Fasted Cardio Benefits vs. Fed Cardio

What is fasted cardio?

Fasted cardio refers to performing cardiovascular exercise while in a fasted state.  Fasted cardio differs from fed cardio in that the body burns fat stores for energy instead of glycogen stores.

Fasted Cardio Benefits (Possibilities)

After reviewing the available research in which the physiologic effects of fasted cardio were documented, I’ve compiled a list of potential health benefits that might be unique to, or solely attainable via, fasted cardio.  Because there are serious limitations associated with most fasted cardio studies, it remains unclear as to whether these prospective benefits are legitimate and/or unattainable with fed cardio.

Appetite suppression: Some evidence suggests that fasted cardio performed in the morning (after an overnight fast of 12 hours) may be superior to fed cardio for reducing 24-hour energy intake.  One study reported that men who performed fasted cardio consumed significantly fewer calories (~3659) in a 24-hour period than men who performed fed cardio (~4682).  While this was a small study, it’s possible that fasted cardio suppresses appetite to a greater extent than fed cardio, leading to lower calorie intake.  If the anorectic effect of fasted cardio is superior to that of fed cardio – it’s reasonable to suspect that this would make it easier for some individuals to lose weight.

Autophagy induction: One health benefit that I suspect is probably unique to fasted cardio (or less likely to be attained and/or less significant via fed cardio) is the induction and/or potentiation of autophagy.  Autophagy is a cellular process whereby cells disassemble unnecessary and/or dysfunctional components.  In other words, during autophagy, the cells within your body are able to metabolize and recycle damaged molecules and organelles; think of this as your cells cleaning up their waste and/or taking out their trash.

Because high rates of autophagy are characteristic of young organisms, and autophagy tends to decrease with age (resulting in the accumulation of cellular damage) – most agree that autophagy is cytoprotective and may delay the aging process.  Moreover, the suppression of autophagy is associated with insulin resistance and hyperinsulinemia – each of which are causally implicated in diabetes and obesity.  To substantially increase autophagy, glycogen stores within the liver necessitate depletion.

Fasted cardio may accelerate the depletion of liver glycogen stores to accelerate the induction of autophagy.  Furthermore, even if a high rate of autophagy is exhibited prior to cardio (such as from an 18-hour fast), it is reasonable to suspect that engaging in fasted cardio might dramatically bolster autophagy rate.  Though this benefit is unsubstantiated with evidence from human fasted cardio trials, it makes logical sense that fasted cardio would enhance autophagy (due to the fact that autophagy is induced by glycogen depletion).  (Source: https://www.ncbi.nlm.nih.gov/pubmed/8238506).

Body fat loss: It is understood that acutely, fasted cardio yields greater body fat loss than fed cardio.  In part, this is because the body is directly burning its fat stores to fuel the cardio workout.  To be clear, superior acute fat loss from fasted cardio does not suggest superior long-term fat loss from fasted cardio – compared to fed cardio.  Nevertheless, one study conducted over the span of ~1 month reported superior fat loss among individuals who performed fasted cardio (in the early evening after a 15-17 hour fast) compared to those who performed fed cardio.

Glucose tolerance: Glucose tolerance refers to how efficiently the cells within your body are able to absorb glucose or sugar after you ingest various foods.  When glucose tolerance is impaired, this increases risk of cardiovascular disease, insulin resistance, type 2 diabetes, and mortality.  Preliminary evidence suggests that fasted cardio might be superior to fed cardio for improving glucose tolerance.

One small study discovered that performing fasted cardio 4 days per week significantly improved whole-body glucose tolerance to a greater extent than fed cardio.  This study was conducted in healthy persons consuming hypercaloric (~+30% kcal/day) and fat-rich (~50% kcals from fat) diets.  Nevertheless, it’s possible that fasted cardio may be superior to fed cardio for improving whole-body glucose tolerance among persons consuming hypercaloric and/or fat-rich diets.

HDL cholesterol: HDL (high-density lipoprotein) cholesterol is colloquially referred to as “good cholesterol” because it generally aids in the removal of harmful forms of cholesterol from the bloodstream.  In most cases (not all cases), increases in one’s HDL cholesterol level is considered healthy.  One study suggests that fasted cardio (performed in the early evening after a 15-17 hour fast for ~1 month) significantly increases HDL cholesterol whereas fed cardio does not.

At the end of the study, HDL levels among participants performing fasted cardio had increased by ~27.3%.  Assuming that the results of the aforementioned study accurately reflect the respective effects of fasted and fed cardio on HDL concentrations, it’s reasonable to state that fasted cardio may effectively improve or favorably modulate cholesterol levels in a subset of persons.

High fat & hypercaloric diet:  Preliminary evidence suggests that fasted cardio may attenuate deleterious health effects of high fat, hypercaloric diets.  A study in which healthy individuals were assigned to perform either: fasted cardio, fed cardio, or no cardio – while consuming high-fat, hypercaloric diets discovered that fasted cardio was significantly more effective than fed cardio (and no cardio) in counteracting diet-induced glucose and insulin abnormalities.  Because consumption of high-fat, hypercaloric diets can lead to a host of medical conditions such as: insulin resistance, type 2 diabetes, cardiovascular disease, and/or metabolic syndrome – it’s possible that fasted cardio would prove therapeutic as an intervention.

Insulin sensitivity: Insulin sensitivity describes how sensitive the body is to the effects of insulin.  More specifically, insulin sensitivity refers to the amount of insulin that’s necessary in order for the body to store glucose.  If you only require a small amount of insulin to store glucose, you’re considered “insulin sensitive,” whereas if you require a significant amount of insulin secretion to store glucose or are unable to effectively store glucose, you may be considered “insulin resistant.”

The higher your insulin sensitivity, the less likely you are to develop type 2 diabetes and metabolic syndrome.  Research suggests that performing fasted cardio may be an effective way to increase insulin sensitivity.  In a study with individuals consuming high-fat, hypercaloric diets, fasted cardio significantly improved insulin sensitivity (or decreased signs of insulin resistance) compared to fed cardio.  This considered, it’s possible that fasted cardio might be better for normalizing sensitivity of insulin among individuals with abnormalities.

Muscle retention:  Proponents of fasted cardio often claim that it is superior to fed cardio for the sake of muscle retention.  Though the evidence to support this claim is weak, one study conducted over the span of ~1 month discovered that fasted cardio led to significant fat loss and body weight reduction, whereas fed cardio solely led to significant body weight reduction.  If the results from the aforementioned study accurately portray the respective effects of fasted and fed cardio, it’s reasonable to infer that fasted cardio would be favorable to fed cardio for lean mass preservation.  (Keep in mind that other studies have found no significant differences in body compositions among adherents to fasted and fed cardio over a 1-month span).

Type 2 diabetes:  For those unfamiliar, type 2 diabetes is a condition in which cells throughout the body are unable to efficiently utilize glucose for energy.  Individuals with type 2 diabetes may exhibit: high blood sugar, insulin resistance, and/or insufficient insulin production.  Because type 2 diabetes can increase mortality and induce a host of deleterious health complications, prevention and/or early treatment are critical.

One study discovered that fasted cardio simultaneously improves whole-body glucose tolerance and insulin sensitivity among persons consuming fat-rich (50% kcal/day from fat) and hypercaloric (~+30% kcal/day) diets, whereas fed cardio did not.  Due to simultaneous improvements in glucose tolerance and insulin sensitivity, researchers suggested that fasted cardio may prove efficacious as a prophylactic or treatment for type 2 diabetes.

Fasted Cardio Disadvantages (Possibilities)

While most research suggests that fasted cardio may benefit general health, other research indicates that fasted cardio likely provides no special benefits that would differ from the benefits of standard fed cardio.  Included below is a list of potential drawbacks or disadvantages associated with performing fasted cardio.

  • Elevated stress hormones: The general consensus from fasted cardio research is that stress hormones and/or neurotransmitters such as: cortisol, epinephrine, and norepinephrine – significantly increase before, during, and after fasted cardio. Some researchers believe that elevated cortisol in particular (as a result of fasted cardio) could increase likelihood of overeating, weight gain, and/or developing medical conditions like hypertension and anxiety disorders.
  • Endurance capacity reduction: It is well-documented that endurance capacity decreases while performing fasted cardio – as compared to fed cardio. If your primary aim is endurance such as marathon running, it’s generally better to perform cardio in a fed state as this should help maximize your endurance capacity.
  • Low energy (Fatigue): When fasted, most people will report lower energy levels than 1-2 hours after being fed. Lower energy levels may translate to mental fatigue, physical fatigue, and/or performance declines during cardio.  Lower energy levels might also lead to significant increases in subjective rates of perceived exertion – especially if the cardio is intense and/or of moderate duration.
  • Lower subjective enjoyment: Many people don’t enjoy performing cardio as much while fasted compared to while fed (e.g. eating a snack 30-90 minutes before). Lower subjective enjoyment from fasted cardio (compared to fed cardio) may be attributable to elevated concentrations of stress hormones and/or neurotransmitters, as well as fluctuations in hunger-related hormones (e.g. ghrelin).
  • Muscle loss: Though some believe that muscle is actually preserved to a greater extent during fasted cardio than fed cardio, preliminary evidence suggests that muscle catabolism is likely to be greater during and after fasted cardio.  In other words, you may end up losing significantly more muscle from fasted cardio than fed cardio – not something you’d want if you’re already low on lean mass and/or are a bodybuilder.
  • Negative health consequences: It’s possible that (in some cases) fasted cardio may be of detriment to a person’s overall health, especially if performed consistently over a long-term. Though most people solely focus on potential upside of fasted cardio – the downside warrants exploration.  Considering the available fasted cardio research, it’s fair to hypothesize that any negative health outcomes associated with fasted cardio might be due to significant increases in the production of stress hormones like cortisol.
  • Poorer performance: It is well-understood that performance may be poorer for moderate-to-high intensity cardio (for a reasonable duration) in the fasted state – compared to in the fed state. Many people will vouch for the fact that cardio performance takes a hit while fasted – as opposed to while fed.  If cardio performance suffers while fasted, your energy expenditure may be less efficient (e.g. you cannot exert yourself as substantially so more time is needed to burn the same number of calories).
  • Side effects or adverse reactions: Fasted cardio is probably not safe for everyone to perform – especially at a high intensity and/or for an extended duration. It is likely that side effects or adverse reactions will occur in a subset of individuals who perform fasted cardio due to preexisting medical conditions or genetic expression.

Note: It should be stated that the above list of hypothetical drawbacks associated with fasted cardio will not apply to everyone.  Some individuals may actually report better performance, higher energy levels, and/or greater subjective enjoyment when doing cardio fasted.  Moreover, variance in genetics, relative caloric intakes, cardio specifics (duration/intensity), and fasting times likely influence the number and/or significance of fasted cardio drawbacks.

Fasted Cardio Requirements (Unofficial)

On the internet, many will argue about the requirements or specific criteria needed to qualify for “fasted cardio,” including: the minimal duration of the fast prior to cardio – as well as whether specific substances can be ingested during the fast without disrupting the physiologic signature of a fasted state.

How long do you need to fast to qualify for “fasted cardio”? (Pre-cardio fasting duration)

Let’s face it, if you finished eating an hour ago and perform cardio, you probably won’t meet the general consensus criteria for fasted cardio (even though you’ve technically been fasted for an hour).  Most would consider the absolute minimum fasting period needed to qualify for fasted cardio as ~8 hours.  In the medical community, many consider 8 hours without food as being the minimal time needed to qualify for a “fast” – this is the duration needed for fasted blood work.

In the exercise and fitness community, most probably consider fasted cardio as performing cardio after a fasting duration of at least 10-16 hours.  Interestingly though, research indicates that a fast for just 6 hours will have optimized fat oxidation for cardiovascular exercise.  So technically, any fast exceeding 6 hours may qualify as “fasted cardio.”

However, because it takes 16-24 hours to fully deplete liver glycogen stores, there may be a subset of individuals who only consider “fasted cardio” as cardiovascular exercise performed after a minimum of a 16-hour fast.  In brief, the longer you’ve been fasted prior to cardiovascular exercise, the more consistent your physiology will be with the fasted state – increasing likelihood that you’ll exhibit physiologic adaptations and corresponding health benefits attained solely via “fasted cardio.”  (In my opinion, the sweet spot for attaining potential health benefits from fasted cardio training is ~16 hours).

Are you allowed to ingest substances during the fast before “fasted cardio”? (Fasting specifics)

It is unknown as to whether the ingestion of substances such as: supplements, medications, and/or certain beverages (e.g. coffee) might modulate physiology in ways that are no longer consistent with a fasted state.  Some speculate that supplements, medications, and/or beverages other than water may activate hepatic enzymes and/or digestive processes, which in turn, might shift physiology in ways that are not consistent with the fasted state.  If this would occur, it’s possible that a subset of potential health benefits associated with fasted cardio might be unattainable.

For this reason, it’s debated as to whether the ingestion of any substance besides water might disrupt fasted physiology and negate certain benefits of fasted cardio.  Some will say that it’s fine to ingest substances like BCAAs (branched-chain amino acids) and caffeine during the pre-cardio fast, whereas others will disagree.  Most experts recommend only consuming water if the aim is to maintain fasted physiology.

What’s better for general health: fasted or fed cardio?

Neither. As of current, research comparing the effects of fasted and fed cardio is limited, therefore, it’s impossible to know whether fasted cardio is somehow superior to fed cardio for long-term general health – or vice-versa.  It’s possible that both fasted and fed cardio facilitate similar overall health benefits.  That said, it’s also possible that there are unique health benefits that can only be attained via fasted cardio – and others that can only be attained via fed cardio.

Moreover, it’s possible that there are negative health consequences solely attributable to fasted cardio, and others solely attributable to fed cardio.  Because neither format of appears inherently superior to the other for the maintenance of general health, I’m of the mindset that whichever form of cardio (fasted vs. fed) yields highest compliance or you as an individual should be regarded as the better option.

  1. Adherence: Most would agree that consistently doing some form of cardiovascular exercise is better than doing no cardiovascular exercise.  For this reason, when deciding whether fasted or fed cardio is “better,” I’d recommend giving each a try for a reasonable duration to determine which format you have an easier time sticking with and/or incorporating into your schedule.  If you find yourself struggling with adherence to a fasted cardio regimen, yet have an easy time adhering to a fed cardio regimen – it’s an obvious choice that you should do fed cardio; vice-versa also applies.
  2. Subjective feeling: If fasted and fed cardio regimens are of similar adherence difficulty, you may want to assess which form of cardio makes you feel better (during and after). If you like the way you feel while performing fasted cardio – you should do fasted cardio.  If you like the way you feel while performing fed cardio – do fed cardio.  Some people may dislike the catecholamine surge that occurs in fasted cardio, whereas others may love it.
  3. Carb vs. fat adapted: Though complete conjecture, it’s possible that individuals who are carb-adapted (burning glycogen for energy) are less suited to perform fasted cardio than persons who are fat-adapted such as in ketosis (burning ketones for energy). The body should be most efficient at burning whatever source of energy that it has adapted to, and for most people, this is carbohydrates.  If a person who is carb-adapted performs fasted cardio, his/her body ends up transitioning from burning glycogen for energy to burning fat.  However, because the person is not fully “fat adapted,” the energy burning during fasted cardio may be less efficient than if they were fat adapted.  This may place the body under greater stress, resulting in muscle catabolism (due to lack of fat adaptation) and/or fewer total calories burned.  On the other hand, a person who is fat-adapted, such as from the ketogenic diet, may efficiently burn calories during fasted cardio (due to preexisting fat adaptation) and may have an easier time preserving lean mass.  Moreover, individuals who are fat-adapted may burn calories with equal efficiency in fasted and fed states (because both involve utilizing fat for fuel).
  4. Data interpretation: Because existing research comparing the effects of fasted and fed cardio is low-quality, many may interpret the data in accordance with preexisting biases. For example, someone who has religiously practiced fasted cardio may have a difficult time admitting that fed cardio may be superior to fasted cardio.  If you’ve always done fed cardio and never enjoyed doing fasted cardio, you’re probably “team fed cardio.”  Anyways, assuming you think that preexisting data from studies comparing fasted and fed cardio are reliable, you may want to choose the format of cardio that appears to facilitate the health effects that you’d personally deem most beneficial.
  5. Genetics: It’s possible that, at some point in the future, researchers will identify specific genes, gene complexes, and/or polymorphisms that make people better suited to perform fasted cardio or fed cardio. Assuming any such genes are ever identified, you’d probably want to choose the cardio format that seems most compatible with your genome.

If you still cannot decide which format of cardio to perform after my suggestions – I’d recommend mixing up your workouts and doing both.  As I’ve said, there may be health benefits that are unique to each cardio-type, and by including both fasted and fed cardio in your regimen, you won’t be missing out.

My thoughts: Which form of cardio (fasted vs. fed) is probably better for most people?

Fed cardio.  As a preface, I should state that I regularly perform both fasted and fed cardio, and actually perform fasted cardio more often than fed cardio.  That said, if I were forced to recommend one format of cardio over the other, I’d probably endorse fed cardio for various reasons underscored below.

  • Lower stress: Fed cardio is associated with a lower stress response than fasted cardio. This means catecholamine concentrations and stress hormones like cortisol will be lower.  If you are prone to bouts of anxiety this format of cardio may exacerbate these feelings.  Additionally, lower levels of cortisol are favorable in regards to appetite control and long-term weight loss.
  • Glycogen-adapted: If you aren’t in ketosis (such as from adhering to a ketogenic diet), I believe that fed cardio is superior to fasted cardio due to the fact that the body efficiently utilizes glycogen stores for energy. If a glycogen-adapted person performs fasted cardio, especially after a long fast, he/she will not be fully “fat adapted.”  This might result in less efficient energy expenditure and possible muscle catabolism during fasted cardio due to lack of fat adaptation.
  • Better workouts: If you’re trying to maximize endurance, speed, and/or overall performance during cardio sessions, you’ll probably want to avoid fasted cardio. Additionally, most people will probably derive greater subjective enjoyment from fed cardio than fasted cardio due to lower stress hormones, lack of hunger, and higher performance.  Also, it’s fair to argue that, for most, cardiovascular workouts will probably be more efficient when performed in a fed state.  Why? Because if you’re fed, your performance will be better whereby you’ll have an easier time burning more calories in a shorter amount of time.
  • Sustained fat loss: While fasted cardio is better for acute fat loss than fed cardio, there probably isn’t any significant long-term fat loss advantage from fasted cardio. In fact, a study that I liked suggested that respective physiologic changes indicate that fed cardio is probably superior to fasted cardio for long-term fat loss.  Specifically, fed cardio increases metabolic rate and decreases respiratory exchange rate for 12-24 hours after a training session and fasted cardio does not.
  • Long-term effects: The long-term effects of regular fed cardio are well-documented and are understood to benefit one’s health. However, the long-term effects of regularly-performed fasted cardio remain unknown.  Some researchers suspect that there could be negative health effects from chronic or regularly-performed fasted cardio due to significant increases in the production of stress hormones (e.g. cortisol).

Am I completely throwing fasted cardio under the bus?

Not at all.  I think there may be some potential benefits derived from the strategic implementation of fasted cardio such as accelerated rates of autophagy, upregulated nerve growth factors (mediated by ketones), and/or other fasted cardio-induced hormetic effects.  There may also be advantages of using fasted cardio (instead of fed cardio) if attempting to improve whole-body glucose tolerance and/or insulin sensitivity.  Furthermore, based on current data, I suspect that fasted cardio might be better for individuals who consume high-fat and/or hypercaloric diets, and/or obese individuals.

Note: I should state that things like fasting specifics (duration and time of day); cardio specifics (intensity, duration, time of day); dietary intake (macros and relative calories); personal attributes (age, body composition, health status, sex); and timing of post-cardio food consumption may affect responses to fed and/or fasted cardio – possibly altering benefits/disadvantages and/or their significances.

Fasted Cardio Research (Summary of Studies)

Included below are all studies that I could find in which the effects of fasted cardio were examined.  Understand that many of these studies sought to determine how fasted cardio influenced specific physiologic biomarkers – rather than attempting to determine how fasted cardio might affect general health.  There were some studies in which the effects of fasted and fed cardio on body composition were compared, however, most studies are full of limitations such that the reliability of the outcomes should be questioned.  I provided a brief synopsis of each study and noted some limitations thereafter.

2016: Effects of aerobic exercise performed in fasted v. fed state on fat and carbohydrate metabolism in adults: a systematic review and meta-analysis.

Vieira, Costa, Macedo, et al. conducted a systematic review and meta-analysis to determine the how aerobic exercise performed in fasted versus fed states influence fat and carbohydrate metabolism in adults.

  • Methods: Researchers searched scientific journal databases for all studies in which the effects of fasted versus fed cardio in adults were documented.
  • Measures: Fat oxidation during exercise; plasma insulin, glucose, and NEFA (pre-exercise and post-exercise)
  • Data included: Researchers found 27 studies that met inclusion criteria for their review and meta-analysis. A cumulative total of 273 participants were involved in the 27 studies for analysis.
  • Results: Fasted cardio significantly increased fat oxidation during exercise compared to fed cardio. Average concentrations of glucose and insulin were significantly higher during fed cardio compared to fasted cardio.  Average NEFA (non-esterified fatty acid) levels did not significantly differ in states of fasted cardio versus fed cardio.

It was concluded that aerobic exercise (cardio) performed in the fasted state induces significantly higher fat oxidation compared to exercise performed in the fed state.

Does this study suggest health benefit from fasted cardio?

No.  This study merely supports the idea that fasted cardio results in greater fat oxidation and lower concentrations of glucose and insulin – compared to fed cardio.  The study does not suggest that fasted cardio is somehow superior to fed cardio for overall health.  Most familiar with metabolism shouldn’t be all too surprised with the result of this review.

When the body is in a fasted state, it oxidizes more fat stores for energy and both glucose and insulin should be lower compared to in a fed state.  Many amateurs mistakenly assume that since more fat is oxidized during fasted cardio, that this somehow translates to more fat loss.  The truth is that while more fat is oxidized during fasted cardio than fed cardio it probably won’t lead to more weight loss than fed cardio – assuming cardio duration/intensity and caloric intakes are identical.

It is possible that there are specific health benefits attributable to high fat oxidation that occurs in fasted cardio that are unattainable via fed cardio.  That said, the high fat oxidation that occurs in fasted cardio might also be deleterious to health over a long-term.  The researchers involved in this review underscore that health consequences resulting from specific biomarker changes in states of fasted cardio versus fed cardio remain unknown and warrant investigation.

(Source: https://www.ncbi.nlm.nih.gov/pubmed/27609363)

2016: Exercising in the Fasted State Reduced 24-Hour Energy Intake in Active Male Adults.

Bachman, Deitrick, and Hillman conducted a study to determine the effect of fasted exercise versus fed exercise in adult men.

  • Participants: 12 active males
  • Design: Randomized, counterbalanced
  • Groups: Fasted versus fed – then 60-minute run at 60% of VO2 max
  • Measures: 24-hour energy intake; blood glucose; hunger; respiratory quotient (during exercise)
  • Results: Fasted cardio resulted in significantly lower 24-hour energy intake (~15312 kJ) compared to fed cardio (~19172 kJ). Respiratory quotient was also significantly lower in the fasted group (~0.86) compared to the fed group (~0.90).  Blood glucose levels were also significantly lower in the fasted group (~4.5 mmol/L) compared to the fed group (~5.2 mmol/L).

It was concluded that fasting prior to morning exercise appears to decrease 24-hour energy intake and increase fat oxidation during exercise.  It was further stated that exercising in the morning while fasting may prove beneficial in weight management.

Does this study suggest health benefit from fasted cardio?

Yes.  The results of this study support the idea that performing morning cardio, prior to consumption of breakfast, may decrease total energy intake throughout the day.  Assuming results accurately reflect the effect of fasted cardio on 24-hour energy intake, it would seem as though fasted cardio might make it easier for people to restrict calories and lose weight.

Limitations of this study

There are some serious limitations associated with this study.

  1. Sample size: The sample size included within the study is very small (12 participants). With such a small sample, it’s possible that the findings do not accurately reflect 24-hour energy intakes in fasted versus fed states; the study is underpowered.
  2. Sample specifics: The sample consisted solely of white male adults who were physically active. It’s unclear as to whether ethnicity, sex, and/or having a history of physical activity may have influenced results.
  3. Duration: The study was conducted for an extremely brief duration (1 day). Had the study continued for a longer-term (e.g. weeks, months, etc.), results may have differed.
  4. Timing: It’s possible that there may be an interaction between the circadian timing of fasted cardio and 24-hour energy intake. Perhaps if fasted cardio were performed in the afternoon (before lunch) or evening (before dinner) results may have differed.

(Source: https://www.ncbi.nlm.nih.gov/pubmed/27738523)

2015: Comparison of the effects of acute exercise after overnight fasting and breakfast on energy substrate and hormone levels in obese men.

Kim, Lee, Choi, et al. examined the acute effects of fasted cardio in 10 obese male college students.  They organized a study in which the students performed fasted cardio for one week (treadmill exercise at 75% of VO2 max – burning a total of 400 kcal) and fed cardio for the following week.  Researchers tracked energy substrate levels and metabolic hormone concentrations to determine changes attributable to measurement times and exercise type (fasted versus fed).

  • Participants: 10 obese college students (BMI > 25 kg/m2 and/or body fat > 20%)
  • Design: Crossover
  • Methods: Morning exercise after overnight fast (first week) vs. morning exercise after breakfast (second week)
  • Duration: 4 weeks
  • Measures: Energy substrate levels (glucose, free fatty acid); metabolic hormone levels (insulin, growth hormone, cortisol) – before exercise, after exercise, and 60 minutes post-exercise.
  • Results: Researchers observed significant interaction effects between measurement times (e.g. before, after, and 60-minutes after – exercise) and exercise type (fasted vs. fed) for concentrations of free fatty acids, growth hormone, and cortisol. Measurement times also significantly affected insulin levels.

It was concluded that fasted cardio performed in the morning (after an overnight fast) might prove more efficacious in reducing body fat than post-prandial (fed) exercise.  That said, researchers suggested that elevated cortisol levels in obese men after fasted cardio performed in the morning (after an overnight fast) may deleteriously influence long-term weight loss efforts.

Does this study suggest health benefit from fasted cardio?

No.  Researchers in this study specifically stated that performing fasted cardio in the morning (after an overnight fast) is more effective for acute body fat reduction when compared to fed cardio in the morning – among obese individuals.  Despite this finding, it is unknown as to whether fasted cardio is superior for reducing body fat over a long-term than fed cardio.

It is well-understood that more fat is acutely burned while performing cardio in a fasted versus fed state – this is an incontrovertible fact.  However, assuming caloric intake is constant (from day-to-day), there probably won’t be superior long-term fat loss attributable to fasted cardio compared to fed cardio.  Why?  Because even if more fat is acutely burned from exercising in a fasted state compared to a fed state, more calories will be stored as fat when the calories are finally consumed later in the day.

Moreover, potential disadvantages of performing fasted cardio were also highlighted by researchers in this study.  The researchers suggested that performing fasted cardio may result in greater muscle wasting during exercise than fed cardio.  In other words, fasted cardio might cause the body to utilize more of its muscle tissue for fuel than fed cardio – likely leading to greater muscle loss among individuals who perform fasted cardio.

Another prospective disadvantage of fasted cardio mentioned by researchers was that it significantly increases cortisol concentrations.  High levels of cortisol have been linked to an array of unfavorable health outcomes, including:  promotion of fat storage, reduced insulin sensitivity, elevated risk of osteoporosis, increased risk of hypertension, immune system suppression, and muscle mass reductions.  Authors of this study suggested that the effects of fasted cardio versus fed cardio on human health, especially over a long-term, remain unknown.

Limitations of this study

  1. Sample size: The sample used for this study was extremely small – just 10 participants. Because the study was underpowered, it’s possible that biomarker measures do not accurately reflect the effects of fasted and fed cardio.
  2. Sample specifics: Only college-aged males who met diagnostic criteria for obesity were included in this study. It’s possible that the outcomes would’ve differed with a sample that included: other age groups, females, and/or non-obese persons.
  3. Timing: In this study, fasted cardio was performed in the morning [following an overnight fast]. It’s possible that if the fasted cardio was performed in the afternoon and/or evening that results may have differed – due to influence of the circadian rhythm.
  4. Duration: The duration of this study was extremely short, such that, it’s difficult to make any conclusions about the long-term effects of fasted cardio on general health.

(Source: https://www.ncbi.nlm.nih.gov/pubmed/26180350)

2014: Body composition changes associated with fasted versus non-fasted aerobic exercise.

Schoenfeld, Aragon, Wilborn, et al. sought to test the hypothesis suggesting that performing fasted cardio in the morning [after an overnight fast] may accelerate fat loss compared to performing fed cardio in the morning.  For the study researchers recruited 20 healthy women following hypocaloric diets and assigned them to one of two groups: FASTED (fasted cardio in the morning) or FED (fed cardio in the morning).  Researchers monitored changes in fat mass and fat free mass among the women over a 4-week duration.

  • Participants: 20 healthy women
  • Design: Randomized, controlled
  • Groups: Fasted cardio vs. Fed cardio
  • Cardio details: 1 hour of steady-state aerobic exercise performed 3 days per week on a treadmill (low-to-moderate intensity).
  • Duration: 4 weeks
  • Measures: Fat mass; fat free mass – change from baseline to endpoint.
  • Results: After 4 weeks, both “fasted” and “fed” groups lost a significant amount of weight and body fat mass.  That said, there were no significant between-group differences in any of the measured outcomes: BMI, body mass, body fat, fat-free mass, fat mass, waist circumference.

Researchers concluded that body composition does not appear to significantly differ among individuals who perform fasted cardio versus fed cardio.

Does this study suggest health benefits from fasted cardio?

No.  Based on the results of this study, there don’t appear to be any health benefits specifically attributable to performing fasted cardio compared to performing fed cardio.  Both fasted and fed cardio are equally efficacious in augmenting a hypocaloric diet for the induction of fat loss and weight loss.

Limitations of this study

Though this study was extremely well-designed, it has limitations that are necessary to mention.

  1. Sample size: The sample size used for this study was small (20 participants). Because the sample was so small, it’s unclear as to whether lack of between-group differences observed at the trial endpoint (after 4 weeks) accurately reflect the effects of each cardio type (fasted versus fed).  Perhaps a larger sample size would’ve discovered between-group differences.
  2. Sample specifics: This study consisted of young women adhering to hypocaloric diets. Perhaps conducting a study with different age groups, males, and/or individuals on maintenance diets (or hypercaloric diets) would’ve generated different results.
  3. Group specifics: The “fed” group (~21 years of age) was significantly younger than the “fasted” group (~23.8 years of age) in this study. Although most would consider this a very modest difference, it’s possible that the statistically significant difference in age between groups might’ve affected outcomes.
  4. Duration: The study was conducted over a span of 4 weeks. While this is a reasonable duration, it may be too brief to discover significant differences of fasted versus fed cardio on body composition.  Moreover, this study is too brief to determine the long-term effects of fasted versus fed cardio on overall health.

(Source: https://www.ncbi.nlm.nih.gov/pubmed/25429252)

2012: Effects of fed- versus fasted-state aerobic training during Ramadan on body composition and some metabolic parameters in physically active men.

Trabelsi, el Abed, Stannard, et al. sought to determine the effects of fasted cardio versus fed cardio on body composition and metabolic parameters during Ramadan in physically active men.  All participants engaged in continuous aerobic exercise at least 3 times per week in the forms of: cycling, running, and rowing for 40-60 minutes at 60-80% maximum heart rate while under supervision.  However, approximately half of the participants were assigned to perform fasted cardio, whereas the others were assigned to perform fed cardio.

Individuals in the fasted cardio group were instructed to abstain from food, supplement, and/or medication consumption for at least 15 hours prior to cardio exercise.  During this study, all exercise sessions were performed in the mid-afternoon from 4:00-6:00 PM (fasted cardio) and from 9:30-10:30 PM (fed cardio).  Researchers noted that RPE (rate of perceived exertion) and exercise session duration did not differ between the groups or change throughout the study.

  • Participants: 19 active men
  • Design: Cross-sectional
  • Groups: Fasted cardio vs. Fed cardio
  • Cardio specifics:
  • Duration: ~1 month (Ramadan)
  • Measures: Body composition; dietary intake; urine specific gravity; serum biochemistry – collected 4 times (before Ramadan, 15th day of Ramadan, 29th day of Ramadan, and 21 days post-Ramadan).
  • Results: Body fat percentage significantly decreased in the “fasted” group by 6.2% (p=0.016), whereas no significant change in body fat occurred in the “fed” group. Body weight significantly decreased in “fasted” and “fed” groups by 1.9% and 2.6%, respectively.  The “fasted” group exhibited increases in various biomarkers including: urine specific gravity (0.64%), urea (8.7%), creatinine (7.5%), uric acid (12.7%), sodium (1.9%), chloride (2.6%), and HDL-C (high-density lipoprotein cholesterol) (27.3%) – at the end of Ramadan compared to baseline.  By comparison, the “fed” group solely exhibited increased creatinine levels (5.8%) at the end of Ramadan compared to baseline.  Both groups exhibited significant reductions in creatinine clearance at the end of Ramadan compared to baseline.

Researchers concluded that fasted cardio simultaneously reduces body fat percentage and body weight, whereas fed cardio solely reduces body weight.  Moreover, fasted cardio yielded substantial changes in a host of biomarkers, nearly all of which were nonexistent in fed cardio.

Does this study suggest health benefit from fasted cardio?

Yes.  The results of this study suggest that fasted cardio is superior to fed cardio for reducing body fat [over a 1-month span].  Both types of cardio yielded significant reductions in body weight – with more weight loss actually resulting from fed cardio than fasted cardio.  However, no significant fat loss occurred in the fed cardio group, whereas significant fat loss occurred in the fasted cardio group.

Considering the lack of significant fat loss in the fed cardio group, we must assume that the fed cardio group burned through more muscle tissue than fat to lose body weight.  Additionally, the results of this study indicate that fasted cardio may facilitate favorable changes in specific biomarkers such as HDL (high-density lipoprotein) cholesterol – something that does not seem to occur after fed cardio.  It was also noted that absolute caloric intake was significantly lower among the “fasted” cardio group, indicating a possible favorable effect of fasted cardio on caloric intake.

Limitations of this study

There are some significant limitations associated with this study that warrant discussion.  Given the numerous limitations listed below, it’s difficult to trust the reliability of this study’s outcomes.  Until these limitations are addressed, we should not be confident in the accuracy of this study’s findings.

  1. Sample size: The sample recruited for this study was small (19 participants). Considering the small sample size, it’s possible that the observed outcomes were attributable to random chance instead of cardio type.  A much larger sample than 19 is necessary to generate results of greater reliability.
  2. Sample specifics: The sample for this study consisted of physically active adult males (ages ~26-28). It’s possible that including individuals of different: age (e.g. elderly), sex (e.g. women), and/or activity level (e.g. sedentary) might’ve yielded different results.
  3. Duration: The study was conducted over a relatively short-term (~1 month). It’s possible that results may not accurately reflect longer-term changes in body composition, biomarkers, and/or general health – as a result of cardio type (fasted versus fed).
  4. Food recording: Participants in this study were instructed to record intake of all food and beverages consumed during the week before Ramadan, as well as 3 days per week during Ramadan. It’s possible that food recordings by participants prior to and/or throughout the study were subject to inaccuracies.  Inaccurate food recording may have explained the outcomes better than cardio type (fasted versus fed).  For more accurate results, researchers should’ve given participants food and recorded it themselves – rather than relied on the self-reporting of participants.  Moreover, because food logs were only kept 3 days per week, it’s unknown as to what caloric intakes might’ve been on the 4 days per week in which calories weren’t recorded.  The 4 unrecorded days might have indicated significant changes in macronutrient intakes and/or caloric intakes whereby outcomes are affected.
  5. Caloric intake: It was noted that daily energy intakes during Ramadan did not differ from pre-Ramadan intakes between groups. That said, for the first half of the study, absolute daily energy intake was significantly greater among the “fed” group compared to the “fasted” group.  It’s possible that significantly greater absolute energy intake for half of the study might explain the insignificant body fat loss exhibited by the “fed” group at the end of the trial.  Then again, it’s possible that fasting and/or fasted cardio led to reductions in absolute daily energy intakes within the “fasted” group.
  6. Macronutrient intake: Macronutrient intakes of the participants were not controlled. It was noted by researchers that protein consumption in the “fasted” group was significantly higher than the “fed” group during the first half of the trial.  Protein is the most satiating macronutrient and elicits the highest thermogenic effect.  This considered, it’s possible that greater protein intake might’ve led to decreased caloric intake and/or greater fat loss in the “fasted” group compared to the “fed” group.  Moreover, fat consumption also was significantly greater among the “fasted” group for the first half of the study than the “fed” group – possibly also increasing satiety to a greater extent and/or modulating hormones to induce greater fat loss.
  7. Exercise timing differences: In this study, exercise sessions were performed in the late afternoon (4:00-6:00 PM) for the fasted group and at night (9:30-10:30 PM) for the fed group. Because exercises were performed at significantly different times between the groups, it’s possible that exercise timing influenced observed outcomes to a greater extent than whether participants were fasted or fed.  Exercise timing may have interacted with the circadian rhythms of participants to significantly influence outcomes.  For more reliable results, all participants should’ve exercised at approximately the same times.
  8. General timing: In this study, fasting was initiated before 11:00 PM and exercise sessions were all performed in the mid-afternoon or at night. It’s possible that adjusting either the fasting times (e.g. morning initiation) and/or exercise times (e.g. morning exercise) might’ve yielded different results.  In brief, there might be a significant interaction between the circadian rhythms of participants and: timing of fasting initiation/cessation and/or timing of exercise initiation/cessation.
  9. Relative caloric intake: While it is known that participants’ caloric intakes pre-Ramadan did not significantly differ from their caloric intakes during Ramadan, we cannot be sure of the extent to which each participant was in a caloric surplus, caloric deficit, or consuming maintenance calories – relative to their current basal metabolic rates.  It’s possible that one group contained more participants in a caloric deficit than the other.  It’s also possible that both groups contained the same number of participants in a caloric surplus, caloric deficit, or consuming maintenance calories – but the extent of the surpluses and/or deficits may have significantly differed, thereby influencing outcomes.
  10. Training load: Researchers admitted that a cross-sectional design makes it impossible to know whether each training group experienced the exact same training load. Previous research indicated that training loads are unlikely to differ in fasted and fed individuals, however, it’s possible that training in a fasted state may significantly increase or decrease training load compared to training in a fed state.  Differences in training load may have influenced observed outcomes.
  11. Modality of measurement: In this study, body fat percentage (%) was estimated using an established algorithm that relied upon skinfold measures.  It’s possible that this particular mode of body fat measurement was subject to inaccuracies.  If inaccurate, it’s possible that the outcomes in this study may have also been inaccurate.  Perhaps incorporating an additional measure of body fat (e.g. DEXA) would’ve helped to strengthen the accuracy of recorded changes in body fat throughout the study.
  12. Hydration status: Researchers admitted that the extent to which participants were hydrated throughout the study may have influenced outcomes. In addition, degree of hydration at the times of body weight recordings may have significantly influenced initial body weights and/or final body weights of participants.  If one group was significantly more/less hydrated than the other when body weights were measured, this may have generated misleading results.
  13. Exercise format: It should be noted that one participant in the “fed” cardio group performed swimming with a heart-rate monitor. Because the sample was so small, it’s possible that allowing even a single individual to perform a different type of exercise than the other participants might’ve influenced results.  In other words, swimming for 40-60 minutes may have altered body composition in different ways than other exercise formats utilized in the study.
  14. Climate: Though unlikely to affect outcomes, it’s possible that conducting this trial in a different climate might’ve yielded different results. Perhaps individuals living in colder climates would lose significantly more body fat from fasted cardio than persons in warmer climates.  It’s also possible that there’s an interaction between climate and cardio type such that fasted cardio may be superior to fed cardio for fat loss in warmer climates and inferior in colder climates.

(Source: https://www.ncbi.nlm.nih.gov/pubmed/22248495)

2011: Exercising fasting or fed to enhance fat loss? Influence of food intake on respiratory ratio and excess postexercise oxygen consumption after a bout of endurance training.

Paoli, Marcolin, Zonin, et al. noted that there’s controversy around the hypothesis that fasted cardio enhances weight loss efforts via stimulation of lipolytic activity, possibly resulting in greater fat loss.  For this reason, the aforementioned researchers conducted a study to identify differences in fat metabolism during fasted cardio and fed cardio.

A total of 8 healthy men were recruited for the study and completed 2 training sessions (in random order) while: fasted (without food for 12 hours) or fed (after a Mediterranean breakfast – 25% protein, 22% carbs, 53% fats – total of 673 kcal).  Physiological parameters of participants were recorded at baseline, as well as after exercise.  Substrate utilization was estimated based on respiratory-exchange ratio (RER) – the ratio between the amount of carbon dioxide (CO2) produced in metabolism and oxygen (O2) used.

  • Participants: 8 healthy men (~27 years of age)
  • Design: Experimental
  • Duration: 2 training sessions (1 fasted vs. 1 fed) with 1 week between sessions.
  • Cardio details: 36 minutes on treadmill at 65% maximum heart rate
  • Measures: Heart rate, VO2 (oxygen consumption), and carbon dioxide production – at baseline and post-training (12 hours and 24 hours).
  • Results: Fed cardio increased VO2 and respiratory-exchange ratio (RER) significantly compared to fasted cardio. Moreover, 12 hours after training, VO2 was still greater and RER was significantly lower as a result of fed cardio compared to fasted cardio.  At 24 hours post-exercise, RER remained significantly lower after fed cardio compared to fasted cardio.

Based on the results of this study, researchers suggested that fed cardio is probably superior to fasted cardio for long-term fat loss.

Does this study suggest health benefit from fasted cardio?

No.  Fed cardio increases metabolic rate and decreases respiratory-exchange rate for 12-24 hours after a training session, whereas fasted cardio does not.  In fact, the study suggests that if long-term fat loss is the goal, it’s advisable to perform cardio in a fed state rather than a fasted one.

Limitations of this study

  1. Sample size: The sample size of this study was extremely small (8 participants). With such a small sample, results may not be reliable.  It’s possible that results might’ve differed with a larger sample.
  2. Sample specifics: The sample consisted of 8 healthy young men. It’s possible that outcomes might’ve differed with the inclusion of: other age groups (e.g. older adults), females, and/or unhealthy individuals.
  3. Timing: Exercise sessions were performed in the morning – either before or after breakfast. It’s possible that there’s an interaction between exercise timing, participants’ circadian rhythms, and fasted versus fed states – that determines energy substrate utilization.  Perhaps performing the same study with afternoon or evening exercise and/or different fasting times (initiation/cessation) may have influenced outcomes.
  4. Limited number of training sessions: A total of 2 training sessions were performed by the 8 participants. It’s possible that the limited number of training sessions may have yielded results that do not accurately reflect the effects of fasted and fed cardio.  Perhaps a larger sample of training sessions (e.g. 10 sessions) would’ve generated different results.
  5. Training details: Researchers noted that the intensity of training may have affected outcomes of this study. Perhaps exercising at a significantly higher or lower intensity (than 65% of maximum heart rate) may affect energy substrate utilization and long-term fat loss differently in fasted and fed states than what was observed in this study.  Moreover, it’s possible that the duration of exercise sessions may have affected outcomes.  Perhaps a duration significantly lesser than or greater than 36 minutes would’ve yielded markedly different outcomes.

(Source: https://www.ncbi.nlm.nih.gov/pubmed/21411835)

2010: Training in the fasted state improves glucose tolerance during fat-rich diet.

Proeyen, Szlufcik, Nielens, et al. mentioned that a fat-rich energy-dense diet is known to cause insulin resistance.  Because fasted cardio is thought to stimulate greater fat oxidation than fed cardio, researchers sought to determine whether fasted cardio was superior to fed cardio for restoration of whole-body glucose tolerance and insulin sensitivity among individuals consuming hypercaloric, fat-rich diets.

  • Participants: 27 healthy males (18-25 years old)
  • Design: Randomized, controlled
  • Groups: Fasted cardio vs. fed cardio vs. control (no training) – 4 days per week.
  • Diet details: Hypercaloric (~+30% kcal/day) and fat-rich (50% of kcal)
  • Duration: 6 weeks
  • Measures: Biomarkers and body weight – pretest and posttest.
  • Results: Body weight significantly increased in the control group (~3.0 kg) and fed cardio group (~1.4 kg), but not in the fasted cardio group (~0.7 kg).  Fasted cardio significantly improved whole-body glucose tolerance and insulin sensitivity (p < 0.05) compared to the control and fed cardio.  Moreover, fasted cardio significantly increased muscle GLUT4 protein (+28%), AMP-activated protein kinase alpha phosphorylation (+25%), fatty acid translocase/CD36, and carnitine palmitoyltransferase 1 mRNA – compared to the control and fed cardio.

Researchers concluded that fasted training appears more effective than fed training for: the induction of favorable musculature adaptations, enhancement of whole-body glucose tolerance, and improvement of insulin sensitivity – while consuming a hypercaloric, fat-rich diet.

Does this study suggest health benefit from fasted cardio?

Yes.  The study suggests health benefit from fasted cardio among individuals consuming hypercaloric (+~30% kcal/day) and fat-rich (~50% kcal from fat) diets.  Specific benefits include: beneficial muscle adaptations, enhanced glucose tolerance, and improved insulin sensitivity.  It was mentioned that fasted cardio may prove useful for the prevention and/or treatment of insulin resistance and type 2 diabetes.

Limitations of this study

  1. Sample size: The sample size used for this study was small (27 participants). Due to the small sample size, it’s possible that the results of the study may not accurately reflect the effects of fasted and/or fed cardio on various biomarkers and/or body weight.
  2. Sample details: The sample consisted solely of active, healthy males ages 18 to 25. It’s possible that the inclusion of females, other age groups (e.g. middle-aged adults), less active individuals, and/or unhealthy individuals (e.g. persons with obesity) may have yielded different outcomes.
  3. Diet specifics: The diets consumed by participants were fat-rich (50% kcal from fat) and hypercaloric (+~30% kcal/day). It’s possible that a diet significantly richer in fat (e.g. 80% kcal from fat) and/or more hypercaloric (e.g. +75% kcal/day) would’ve generated different outcomes.  It’s also possible that other diets (e.g. hypocaloric, maintenance calorie, protein-rich, carb-rich, etc.) would’ve yielded different results.
  4. Control feeding: The 7 individuals in the control group were relatively more overfed (~30%) compared to the fasted and fed cardio groups. Researchers note that, because the control group was overfed, it’s impossible to know whether different outcomes were mostly attributable to the exercise interventions or the magnitude of overfeeding.
  5. Fasted group body weight: The body weight of participants in the fasted group did not significantly change throughout the study. Researchers mention that the unchanged body weight in the fasted group may have accounted for some of the favorable outcomes observed.
  6. Fed group macros: The fed cardio group received a carbohydrate-rich breakfast (675 kcal – 70% carbs, 15% fat, 15% protein) approximately 90 minutes before each training session. It’s possible that the specific calorie intake (675 kcal) and/or the macronutrient profile assigned to the cardio group influenced observed outcomes.

(Source: https://www.ncbi.nlm.nih.gov/pubmed/20837645)

2005: Effect of moderate incremental exercise, performed in fed and fasted state on cardio-respiratory variables and leptin and ghrelin concentrations in young healthy men.

Zoladz, Konturek, Duda, et al. hypothesized that performing fasted cardio may stimulate significant changes in plasma concentrations of leptin and ghrelin, which in turn, might alter cardio respiratory function in distinct ways from cardiorespiratory function observed after fed cardio.  For this reason, the aforementioned researchers organized a study to evaluate the effect of fasted cardio (morning cardio after overnight fasting) on cardiorespiratory responses and hormone secretion in healthy adult men.

  • Participants: 8 healthy men (~23 years of age)
  • Design: Experimental
  • Duration: 2 incremental exercise tests (in random order) performed up to 150 W (~59% VO2 max) – once in the fed state and the other in the fasted state (with 1 week between sessions).
  • Measures: Cardio respiratory function; biomarkers (via blood samples)
  • Results: Heart rate was significantly lower during fasted cardio compared to during fed cardio.  Respiratory quotient and plasma lactate levels were significantly lower from fasted cardio than fed cardio.  Plasma gastrin levels were significantly lower in fasted cardio compared to fed cardio.  Plasma IL-6 levels and norepinephrine levels were significantly higher from fasted cardio compared to fed cardio.

It was concluded that leptin and ghrelin concentrations do not induce heart rate reductions observed in fasted cardio.  Researchers suspect that heart rate is lower during fasted cardio (compared to fed cardio) due to increased plasma norepinephrine, which in turn, increases systemic vascular resistance and baroreceptor-mediated vagal stimulation.  In sum, elevated norepinephrine, reduced respiratory quotient, and increased IL-6 levels likely physiologic adaptations that maintain energy homeostasis in fasted states.

Does this study suggest health benefit from fasted cardio?

No.  Researchers neither reported upon nor suggested that fasted cardio generates health benefits that cannot be attained from fed cardio.

Limitations of this study

  1. Sample size: The sample included for this study was extremely small (8 participants). Perhaps the small sample size generated unreliable data.
  2. Sample specifics: The sample consisted of healthy, young adult males. Perhaps including females, other age groups, and/or unhealthy individuals might’ve generated different results.
  3. Timing: Participants in this study fasted overnight and exercised in the morning. Because circadian biology affects hormone secretion, it’s possible that adjusting fasting times (e.g. daytime fasting) and/or exercise times (e.g. nighttime exercise) would’ve altered biomarker changes associated with fasted cardio.
  4. Duration: This was a very short-term study that solely examined 8 participants over the span of 2 training sessions. This considered, it’s possible that a longer-term study with more training sessions might’ve generated different outcomes.

(Source: https://www.ncbi.nlm.nih.gov/pubmed/15795476)

1986: Metabolic responses to exercise after fasting.

Dohm, Beeker, Israel, et al. noted that performing fasted cardio increases fat metabolism and decreases the speed at which glycogen is depleted from muscles.  The aforementioned researchers organized an experiment to determine the effect of fasting on blood metabolite levels during exercise.

  • Participants: 9 males
  • Design: Counterbalanced
  • Cardio details: Running (70% VO2 max)
  • Duration: 2 trials – “fed” vs. “fasted” (23-hour fast)
  • Measures: Biomarker levels (blood test)
  • Results: Fasted cardio significantly increased fat mobilization, fat utilization, and lactate levels – compared to fed cardio.

Researchers concluded that, while fasting, blood glucose levels remain within the normal range during exercise – despite liver glycogen depletion.  At the time of publication, researchers suggested that maintenance of normative blood glucose levels during fasted cardio was probably attributable to increased gluconeogenesis and decreased muscle-glucose utilization.

Does this study suggest health benefit from fasted cardio?

No.  Researchers specifically sought to evaluate whether fasted cardio (after a 23-hour fast) significantly alters blood glucose concentrations during exercise – compared to fed cardio.  No measures were collected with the intent to determine if fasted cardio might be superior to fed cardio (or vice-versa) for long-term general health.

Limitations of this study

  1. Sample size: The sample size used for this study was extremely small (9 participants). With such a small sample size, it’s impossible to know whether the findings accurately portrayed the effects of fasted cardio.
  2. Sample specifics: The sample consisted solely of 9 males. It’s possible that the effects of fasted cardio might’ve been significantly different in females.
  3. Duration: This study was extremely short-term in that it evaluated the effects of just 2 training sessions. Perhaps a longer-term study with a greater number of training sessions would’ve generated different results.
  4. Measures: No measures in this study were collected with the intention of identifying potential health effects of fasted and/or fed cardio. Perhaps different measures could’ve been implemented to help researchers understand potential health benefits of fasted cardio.

(Source: https://www.ncbi.nlm.nih.gov/pubmed/3536834)

Limitations associated with the research of fasted cardio

Included below is a list of limitations I’ve compiled that are associated with research of fasted cardio.  Until these limitations are addressed, it’ll remain difficult to elucidate any potential health benefits (or drawbacks) that are attributable to fasted cardio.

  1. Sample sizes: Most sample sizes utilized to research fasted cardio are very small, including less than 20 participants. The problem with such small sample sizes is that they are more likely to yield unreliable data; most would refer to these studies as being “underpowered.”  To generate more reliable and/or quality data associated with fasted cardio, it is necessary to conduct studies with larger sample sizes.
  2. Study durations: The durations of most studies investigating fasted cardio are relatively short-term. While some studies have been conducted for modest durations of 4 to 6 weeks, others are much shorter (e.g. 2 training sessions).  With limited study durations, it’s difficult to hypothesize any long-term health effects associated with fasted cardio.  Moreover, it’s possible that short-term studies do not provide adequate time to observe significant changes in body composition and/or specific biomarker quantities.
  3. Sample specifics: A majority of fasted cardio studies utilized samples with specific, yet homogenous participant demographics: young adults; physically active; healthy (devoid of medical conditions), etc. While it’s important to clarify the effects of fasted cardio in healthy adult populations, the findings of many available studies may not accurately reflect the effects of fasted cardio in other populations.  For example, there may be different effects of fasted cardio in: older adults; different ethnicities; obese individuals; persons with certain medical conditions; and/or sedentary individuals.
  4. Fasting specifics: Not all studies are consistent with their fasting durations and/or times. For example, the Ramadan study investigated the effects of a fast beginning at 11 PM and ending in the mid-afternoon/evening, whereas other studies initiate the fasting much earlier than 11 PM.  Moreover, the total number of hours fasted prior to performing cardio is not equal in all studies – potentially influencing outcomes (body composition, biomarker levels, etc.).
  5. Measures: Most studies testing the effects of fasted cardio utilize different measures. Because measures are inconsistent from study-to-study, it remains difficult to compare and/or contrast findings.  Additionally, studies in which the effects of fasted cardio on body composition are reported may be subject to inaccurate data based on modality of measurement.  A single measurement error of participant’s body fat in a small-scale fasted cardio study could compromise the entire data set and corresponding conclusions.  Furthermore, measures may not be collected frequently enough in terms of: dietary intakes, body weight, and/or body fat.  Infrequent measurements in body weight for example may produce misleading results simply due to overhydration or dehydration of a participant.  Finally, some studies are basing results on self-reporting data of participants.  Because self-reporting is notoriously inaccurate, it’s possible that any studies relying on self-reports produced inaccurate results.
  6. Diet specifics: Some studies did a great job at ensuring that the macronutrient profiles and relative caloric intakes of participants were identical. Other studies failed to ensure that macronutrient profiles and relative caloric intakes (e.g. ~10% hypocaloric based on BMR) were equal among participants.  Specific diets and/or failing to ensure that all participants followed the same diets – make it difficult to know whether the effects observed in fasted cardio research are attributable to fasted cardio.
  7. Exercise specifics: Many studies tested the effects of fasted cardio in the morning performed at moderate intensity for 40 to 60 minutes. Usually, the cardio was performed at a frequency of several times per week on a treadmill.  In some cases, the exercise was supervised to ensure compliance with the study regimen.  Nevertheless, due to differences in exercise specifics (time of day; duration; intensity; frequency (times per week); mode of exercise; and supervision – it’s difficult to know whether effects of fasted cardio may vary in accordance with exercise specifics.

Considering the research, are there any special benefits solely attained from fasted cardio?

If we were to assume with 100% certainty that the results of existing fasted cardio studies are accurate, it’s reasonable to conclude that there are special benefits attained from fasted cardio that cannot be attained with fed cardio.  However, because we cannot be 100% certain that the results of fasted cardio studies accurately reflect the impact of fasted cardio (due to many limitations), nobody can legitimately claim that there are proven health benefits solely attainable from fasted cardio.  Nevertheless, if we selectively highlight specific health benefits from studies in which fasted cardio was of greater usefulness than fed cardio – below is a summary of those benefits.

  • Decreased caloric intake: Fasted cardio performed in the morning before breakfast (after an overnight fast) may reduce average caloric intake for the remainder of the day compared to fed cardio performed at the same time. Decreased caloric intake from fasted cardio may lead to weight loss, lower inflammation, and improved body composition.
  • Improved glucose tolerance: Fasted cardio performed by individuals on high-fat/hypercaloric diets may significantly improve whole-body glucose tolerance compared to fed cardio.
  • Increased insulin sensitivity: Fasted cardio performed by individuals on high-fat/hypercaloric diets may significantly improve insulin sensitivity.
  • Greater fat loss: One study concluded that fasted cardio performed in the early evening induced greater fat loss than fed cardio performed at the same time – over a 1-month span.
  • Superior muscle retention: One study reported that fasted cardio performed in the early evening induced greater fat loss, but less total weight loss than fed cardo performed at the same time – over a 1-month span. This result suggested that fasted cardio may preserve muscle (lean mass) more effectively than fed cardio.
  • Increased HDL cholesterol: One study reported that individuals adhering to fasted cardio regimens exhibited significant increases in HDL cholesterol whereas persons adhering to fed cardio regimens did not.

Recommendations for future fasted cardio studies

After reviewing all studies of fasted cardio and determining their respective limitations, there are recommendations I’d make for anyone interested in conducting additional fasted cardio research.  Most of these recommendations are obvious, however, some may be difficult to address, especially with limited financial resources.  Nevertheless, in order to accurately to pinpoint specific health outcomes (positive or negative) attributable to fasted cardio, limitations need to be addressed – hence these recommendations.  I’ve also made a few additional suggestions for measures or variables that may prove useful to investigate.

  • Controlled diets: Future studies should aim to control the diets of all participants such that they do not significantly differ among participants. This could be done in a metabolic ward experiment or perhaps by utilizing prepackaged meals.  Another possible way to control diets of participants is to recruit participants with a history of accurate food tracking.
    • Macronutrient profiles: Researchers should ensure that macronutrient profiles of diets are identical among all participants enrolled in fasted cardio studies (e.g. 25% proteins, 30% fats, 45% carbs). Obviously if attempting to compare the effects of fasted and fed cardio among persons adhering to ketogenic diets and high-carb diets – we could divide participants in half and assign specific keto macros to half and specific high-carb macros to the other half.
    • Caloric intakes (relative): Moreover, researchers should ensure that relative caloric intakes of participants (in relation to basal metabolic rates) are identical.  In other words, some participants shouldn’t be hypercaloric with others hypocaloric or at maintenance calories – differences in relative caloric intakes may affect outcomes.
    • Water intakes (relative): Though some might consider this overboard, it may be useful to control relative water intakes of participants throughout the study. Some researchers have admitted that varying degrees of hydration among participants may have affected results of fasted cardio studies.
      • Metabolic ward: The best way to control dietary intakes of participants is to conduct a metabolic ward experiment with fasted cardio whereby researchers give each participant the same amount of food relative to their current basal metabolic rate (e.g. 5% hypocaloric).
      • Prepackaged meals: Another potential strategy might be to format prepackaged meals and/or nutritionally-complete shakes for each experiment – instructing each participant to only consume the food they’ve been given by researchers.
      • Regular food trackers: A less expensive strategy might be to recruit individuals with a history of accurate food reporting (e.g. bodybuilders, dieticians, or fitness professionals) – and instruct them to consume specific macros and relative calories.
  • Controlled exercise: Future studies of fasted cardio should control the exercise-type, intensity, and duration of participants. It is known that different types of exercise for different durations and/or at different intensities may affect study outcomes.  To conduct a study in which the specifics of exercise are controlled, it is recommended that exercise be performed in supervised settings (under professional supervision).  Supervision will ensure that participants are using the same type of exercise (e.g. treadmill) for the same duration (e.g. 30 minutes) at the same intensity (e.g. 60% VO2 max).  Additionally, exercise should be performed at the same frequency throughout the week (e.g. 3 times per week – Monday, Wednesday, Friday).
  • Large sample size: The most obvious way to improve future studies of fasted cardio is via recruitment of a large sample. The samples recruited for most of the existing research have been very small.  Though it’s possible that the small samples yielded results that accurately showcase the effects of fasted cardio, larger trials are needed to substantiate preexisting fasted cardio studies.  I’d recommend recruiting a sample of at least 100 individuals for additional studies.
  • Long-term trial: For a fasted cardio study, I am of the opinion that longer is better. Why?  Longer-term studies would be of greater accuracy in portraying the prospective long-term health effects of regular fasted cardio – something that most people want to know.  If fasted cardio is beneficial or deleterious over a long-term would be good to know.  Moreover, longer-term trials generally collect more data from measures.  The more data collected, the greater the odds that observed biomarker changes are due to fasted cardio – instead of random chance.
  • Measures: Researchers of fasted cardio should endeavor to utilize similar measures to track changes in biomarkers and body composition over time. Additionally, I’d recommend collecting measures as frequently as possible to ensure that outcomes aren’t affected by confounds such as degree of hydration at baseline or endpoint.  If measures are only collected a couple times (e.g. baseline and endpoint), it’s possible that a measure like body weight may be significantly affected as a result of dehydration or overhydration in a subset of participants.  Furthermore, in order to decrease likelihood of measurement errors for body composition, it is recommended to utilize multiple measures together such as: DEXA scan plus calipers to measure body fat.  Finally, in regard to measures, I think it would be of value to determine the effect of fasted cardio on rates of autophagy – as higher rates of autophagy are generally associated with health benefit.
  • Participants: Further studies should attempt to recruit participants of similar ages, ethnicities, health status, sex, and body composition (i.e. body fat, lean mass, etc.). Differences within a sample such as obese vs. non-obese, male vs. female, young vs. old, Caucasian vs. Hispanic – might produce different results.  Researchers should realize that the effects of fasted cardio may vary based on the aforementioned participant-specific variables.

Note: Conducting quality “fasted cardio” studies is somewhat difficult.  It is unlikely that most researchers will be able to account for all of my recommendations.  Nonetheless, those interested in fasted cardio should strive to enhance the quality of research by addressing as many limitations as possible.

Fasted cardio research: Variables for researchers to investigate and manipulate

There are a variety of variables that can affect outcomes of fasted cardio studies.  Some have proposed that fasted cardio may be more beneficial among individuals with obesity, type 2 diabetes, and/or insulin resistance.  Others have suggested that fasted cardio may facilitate different physiologic changes when performed at a high intensity compared to a lower-to-moderate intensity.  Included below are variables that can be manipulated to determine if fasted cardio may be more or less beneficial in a specific demographic.

  • Age: Fasted cardio may have different effects in teenagers, young adults, middle-aged adults, and/or elderly adults. It may be worth determining how fasted cardio affects different populations based on age.
  • Autophagy: A well-known benefit of fasting (and intermittent fasting) is upregulated autophagy, or cellular self-cleaning. Unfortunately, at the time of me writing this article, no studies have examined the effect of fasted cardio on autophagy-related biomarkers.  I hypothesize that fasted cardio might dramatically accelerate the onset of autophagy, as well as autophagy rates – compared to standard intermittent fasting and fed cardio.
  • Body composition: It’s possible that fasted cardio exerts different physiologic effects in obese persons compared to healthy weight or underweight individuals. In addition, it’s possible that fasted cardio elicits different effects based upon a person’s preexisting body fat percentage and lean mass.
  • Cardio performance: It is understood that for endurance and/or high-intensity exercise performance, fed cardio is superior to fasted cardio. Nevertheless, it may be useful to further investigate ways in which cardio performance is affected in fasted and fed states.  For example, it would be of interest to know the approximate intensity and/or duration at which performance significantly declines among persons engaged in fasted and fed cardio, respectively.
  • Cognition: It is known that regular cardiovascular exercise supports healthy brain function and enhances cognitive performance. That said, no studies have attempted to determine whether fasted or fed cardio might enhance cognition to a greater extent (or in different ways) than the other.  Because ketone uptake significantly increases in fasted cardio, and ketones might elicit different effects upon nerve growth factors in the CNS compared to fed cardio – it’s reasonable to suspect that each cardio-type could exert unique effects on cognitive function.
  • Compliance: No studies that I’m aware of have sought to compare general compliance among individuals assigned to perform fasted and fed cardio. It’s possible that general compliance would be significantly greater among persons recommended to perform optional fed cardio compared to those recommended to perform optional fasted cardio.  When it comes to exercising for general health, compliance with exercise regimen is probably the biggest predictor of long-term health benefits.
  • Diet: Different macronutrient profiles (carbs, fats, proteins) and/or relative caloric intakes (e.g. 10% hypercaloric) may interact with the effect of fasted cardio on physiology. Manipulating macros and/or relative calories could help researchers identify the specific ways in which fasted cardio interacts with diet to facilitate physiologic changes.  Moreover, this could help identify specific physiologic changes that are consistent with fasted cardio regardless of diet.
    • “Carb” vs. “Fat” adapted: It would be interesting to compare the respective physiologic effects of fasted and fed cardio among individuals following ketogenic diets and high-carb diets, respectively.  We may find that fat-adapted individuals derive more benefit from fasted cardio than carb-adapted persons or vice-versa.
    • Uncontrolled or partially-controlled diets: It may be worth investigating the effects of fasted and fed cardio among individuals on uncontrolled or partially-controlled dietary regimens. This may help researchers better understand how different forms of cardio might affect 24-hour energy intakes, eating schedules, and/or macronutrient preferences.  For example, we might find that fasted cardio is more likely to decrease 24-hour energy intake and increase preference for carbohydrates when compared to fed cardio among individuals who aren’t adhering to strict diets (like a majority of the population).
  • Epigenetics: As of current, no studies have evaluated epigenetic biomarkers among individuals performing fasted and fed cardio. It’s possible that unique epigenetic changes may occur that are contingent upon cardio-type.  In other words, epigenetic expression may change in different ways from fasted cardio compared to fed cardio.  Moreover, it’s possible that certain epigenetic changes attributable to one cardio type may be superior for general long-term health compared to those attributable to the other.
  • Exercise duration: Performing fasted cardio for different durations may affect physiology in unique ways. (Perhaps longer fasted cardio sessions produce greater or less health benefit than shorter fasted cardio sessions).
  • Exercise intensity: The intensity of exercise might influence the effects of fasted cardio on physiology. Researchers could seek to determine whether low intensity fasted cardio yields different physiologic changes than moderate or high intensity fasted cardio.
  • Exercise frequency: Some studies have documented the effects of fasted cardio performed 3 days per week. Perhaps adjusting the frequency of fasted cardio to 1 day per week (less frequent) would yield different outcomes than 7 days per week (more frequent).
  • Exercise type: The type of exercise performed may affect physiologic responses to fasted cardio. In other words, swimming for 30 minutes at 50% VO2 max may facilitate different physiologic responses than running for 30 minutes at 50% VO2 max.
  • Fasting durations: The duration of fasting prior to engagement in cardio may influence outcomes of studies. For example, fasting for just 8 hours prior to cardio might yield different physiologic effects than fasting for 16 hours prior to cardio.
  • Genetics: It must be stated that genetics may determine how each individual reacts to fasted cardio. Perhaps there are specific genes that make someone more or less likely to derive health benefit (or health detriment) from fasted cardio.
  • Medical conditions: The medical statuses of participants in fasted cardio studies may influence outcomes. For example, specific changes in biomarkers and corresponding health benefits of fasted cardio may only be evident (or most significant) in persons with conditions such as:  obesity, type 2 diabetes, insulin resistance, hyperlipidemia, or hypertension.  It’s also possible that certain medical conditions may be worsened by fasted cardio.  Moreover, fasted cardio may have different effects in healthy persons compared to those with medical conditions.
  • Nerve growth factors (NGFs): It is thought that fasted cardio may activate nerve growth factors that cannot be activated with fed cardio. Why? Because certain nerve growth factors are only upregulated when stimulated by ketones.  Ketone levels significantly increase after a lengthy fast, and increase significantly more if fasted cardio is combined with the lengthy fast.  Considering that ketone-mediated activation of nerve growth factors may yield significant health benefit, it may be useful to examine the effects of fed and fasted cardio (with various pre-cardio fasting durations) on the expression of nerve growth factors.
  • Timing: Fasted cardio may be more beneficial when the fast is performed overnight and the cardio is performed in the morning – as opposed to fasting during the day and exercising at night. It is likely that the timing of both the fasted cardio and the fast interact with circadian biology of participants.  For this reason, manipulating the times of fasted cardio (morning, afternoon, night) and the fasting (daytime vs. nighttime) might alter the physiologic effects and health benefits/detriments of fasted cardio.
  • Sex: Though it doesn’t seem as though there are sex-specific effects of fasted cardio, it’s possible that there are. For this reason, it may be worth conducting fasted cardio studies in solely males, then solely females to elucidate sex-specific biomarker changes.  It may turn out that fasted cardio is healthier or unhealthier in one sex compared to the other.
  • Uncontrolled or partially-controlled exercise: It may be worth exploring how frequently individuals on uncontrolled exercise regimens (i.e. no exercise is required) actually exercise if they must do either: fasted cardio or fed cardio.  Perhaps individuals only allowed to perform fasted cardio would exercise less frequently than persons only allowed to perform fed cardio.  Additionally, it may be useful to investigate how respective fasted and fed cardio affect exercise performance in partially-controlled settings.  For example, participants may be supervised and told to run on a treadmill for 3 times per week with the intent of burning as many calories as possible.  However, because intensity and duration of exercise remain uncontrolled, we may learn whether fasted and fed states yield differences in intensity and/or duration.  It may turn out that persons performing fed cardio burn more total calories (on average) than persons who perform fasted cardio – and vice-versa.  Because a majority of the populace doesn’t adhere to a specific exercise regimen, determining whether fasted or fed cardio may yield superior preference-based energy expenditure during exercise may be useful.

FAQs: Fasted Cardio Benefits

Below are some questions that people may have about fasted cardio.  I’ve endeavored to answer these questions as accurately as possible based on a combination of findings in the literature and common sense.  I’ve also outlined my personal fasted cardio routine and subjective experiences with fasted cardio (for those that are interested).

How long should you fast before fasted cardio?

Depends. It remains unclear as to what the ideal fasting length might be to attain maximal health benefit from fasted cardio.  Assuming there are health benefits that can only be attained from fasted cardio (but not fed cardio), it is likely that the longer the fast prior to the cardio – the better.  The longer a person fasts prior to performing fasted cardio, the less energy he/she will have for the cardio and more likely he/she may experience unwanted side effects or adverse reactions associated with physical exertion in the fasted state.

Consider the following:

  • Cardio duration
  • Cardio intensity
  • Preexisting conditioning
  • Body composition
  • Medical conditions

If you’re wondering how long to fast before you perform fasted cardio, consider the factors listed above.  If the cardio duration will be relatively long and/or the intensity will be high, an extremely long fast prior to the fasted cardio (e.g. 24 hours) may increase likelihood of adverse reactions (e.g. fainting).  On the other hand, if the cardio duration will be relatively short and/or the intensity will be low, a long fast prior to the fasted cardio may be perfectly fine.

For the sake of general safety and/or health of individuals performing fasted cardio, the duration and intensity should be adjusted in accordance to the length of the pre-cardio fast.  Cardio should be less intense and shorter duration when the pre-cardio fast is long, whereas cardio can be more intense and longer duration when the pre-cardio fast is short.  Moreover, a person’s preexisting physical conditioning, body composition, and/or medical conditions should be taken into consideration when attempting to determine fasted cardio duration and intensity.

That said, assuming there are significant health benefits that can only be attained from fasted cardio, I suspect that the longer a person fasts prior to the cardio – the more likely he/she will attain those benefits.  The study in which fasted cardio decreased 24-hour energy intake incorporated a fasting window of 12 hours, whereas the study in which fasted cardio significantly reduced body fat incorporated a fasting window of 15 to 17 hours.  Based on available research, it seems as though a fasting window between 12 and 18 hours should be sufficient to attain fasted cardio-specific health benefits.

Still, some researchers point out the fact that fasting for just 6 hours optimizes fat burning for cardiovascular exercise.  This means that a fast for only 6 hours probably would allow you to attain health benefits that are unique to fasted cardio.  Nevertheless, although longer fasting windows such as 18-24 hours haven’t been investigated in fasted cardio studies, it’s possible that there are specific health benefits that can only be attained from fasted cardio after longer fasts (due to the fact that liver glycogen stores aren’t depleted until ~1 day in the fasted state).

What is the best time of day to perform fasted cardio?

Unknown. (There may not be a universal “best time”).  As of current, it is unknown as to whether there might be an optimal time of the day to perform fasted cardio.  Some suspect that performing fasted cardio at certain times of the day such as the morning may yield superior health benefits than performing it at other times – possibly due to influence of the circadian rhythm.  It is thought that performing fasted cardio in the morning facilitates maximum lipolysis (fat burning) compared to performing fasted cardio at other times of the day.

However, even if lipolysis is significantly enhanced from morning fasted cardio – compared to afternoon or evening fasted cardio, it remains unknown as to whether this would somehow provide significantly greater health benefit.  Perhaps enhancement of lipolysis via morning fasted cardio would somehow be of greater health detriment than fasted cardio in the afternoon or evening.  Of all available studies in which the effects of fasted cardio were investigated, most employed morning fasted cardio.

One study in which fasted cardio was performed in the morning reported that it significantly reduced 24-hour energy intake compared to fed cardio.  Based on this study, most would hypothesize that there might be substantial benefit from morning fasted cardio if the goal is to cut calories and lose weight.  Nevertheless, another study in which fasted cardio was performed in the early evening reported that it significantly reduced body fat over a ~1-month period compared to fed cardio.

Because no study has ever compared the effects of fasted cardio performed at different times of day (morning, afternoon, night) – it is unclear as to whether performing fasted cardio at a specific time of day may yield greater health benefit than performing it at another time.  It’s fair to speculate that there might be time-specific benefits and/or detriments associated with fasted cardio.  Moreover, because circadian biorhythms vary among the general population (based on genetic and environmental factors), it’s possible that the best time of day to perform fasted cardio for health benefit is contingent upon the specific person performing it.

Do any supplements or drinks break the fast before fasted cardio?

Unknown.  Researchers of fasting in humans and animals suggest that the ingestion of any substances other than water while fasting might induce physiologic changes to end the physiologic signature of the fasted state.  In other words, taking medications, supplements, and/or drinking beverages like coffee while fasted might activate enzymes implicated in metabolism and/or digestive processes to alter or disrupt physiology of a fast.

If you do cardio after your physiology has been altered via ingestion of a substance, it’s possible that you will no longer get health benefits that can only be attained through fasted cardio.  For this reason, if you believe there are specific benefits that can only be attained through fasted cardio – it’s probably best to only consume water prior to the cardio session.  Though certain substances may not disrupt the physiology of fasted states, it’s best to err on the side of caution until more research is published.

How soon should you eat after fasted cardio?

Depends. It’s up to you to decide when you want to eat after performing fasted cardio.  In most research of fasted cardio, individuals consumed food either immediately or shortly after the end of their cardio sessions.  Although not necessary to eat immediately after fasted cardio, I personally think it’s a good idea to consume food as soon as possible after fasted cardio if the pre-cardio fast was of reasonable duration – preferably within a 30-minute window.

During and after the performance of fasted cardio, catecholamine concentrations will be elevated – the body is stressed from a combination of being fasted and cardio.  Furthermore, the body will probably be in a catabolic state after fasted cardio such that it’s burning a combination of fat and muscle for energy.  Because food consumption should help reduce sympathetic tone and corresponding catecholamine concentrations, as well as attenuate muscle catabolism – I believe it’s best to consume food immediately after fasted cardio.

That said, if your fast prior to fasted cardio was somewhat short, you may actually want to abstain from food consumption for a few hours after fasted cardio to extend your fasting window.  For example, if you fasted for 12 hours, then did 30 minutes of fasted cardio, you may want to avoid food for another 3 hours to increase ketone circulation and/or autophagy rates – assuming you believe these provide health benefits.

Are there any dangers of fasted cardio?

As I’ve already mentioned, a combination of factors such as: cardio duration; cardio intensity; preexisting conditioning; body composition; and medical conditions – will determine if fasted cardio is dangerous.  Someone with a history of hypotension who performs intense fasted cardio after a 24-hour fast may end up fainting due to a sudden drop in blood pressure.  If you’re unsure about whether fasted cardio is safe for you, it is recommended to ask a medical doctor and discuss the details of your fasting (length) plus cardio (duration/intensity)

It is also necessary to mention that hydration status and electrolyte levels may increase or decrease risk of adverse reactions to fasted cardio.  For example, someone who is dehydrated and/or exhibits electrolyte imbalances could end up cramping up and/or fainting during fasted cardio.  The bottom line: Never perform fasted cardio unless you’re certain it’s safe and you’ve taken proper precaution (such as by staying hydrated).

Do I ever perform fasted cardio?

Yes.  As I’ve mentioned earlier, I perform fasted cardio far more regularly than fed cardio (most of my cardio is done while fasted).  That said, before actually taking time to read the fasted cardio research, I sincerely believed that fasted cardio was undoubtedly superior to fed cardio for general health.  Why? Because on various podcasts I listened to between 2015-2016, many hosts and/or guests that I deemed trustworthy had hyped up the benefits of fasted cardio in terms of brain function, fat loss, and longevity.

One such podcast, SmartDrugSmarts (#120) featured Mark Mattson, a neurologist at Johns Hopkins University and the National Institute of Health (NIH).  Because Mattson endorsed fasted cardio, and because I had already been performing fasted cardio exclusively for the past year or so, my preference for training while fasted was strengthened.  If you listen to the podcast, Mattson implies that fasted cardio is likely of substantial benefit to humans based on its ability to upregulate ketones.

When we fast for 16 hours, Mattson iterated that glycogen stores are depleted within the liver.  Following the depletion of hepatic glycogen, fats are mobilized and converted to ketones, an alternative source of energy to glycogen.  Ketones are known to elicit a host of unique physiologic effects within the brain and nervous system that are unattainable in glycogen-rich states.

If cardiovascular exercise is performed in a 16-hour or more fasted state, Mattson suggests that ketone circulation will be upregulated to a significantly greater extent than it would from just fasting.  In other words, fasted cardio accelerates the production and circulation of ketone bodies, and in turn, these ketone bodies improve one’s health.  Specifically, the ketone bodies are thought understood to upregulate various nerve growth factors throughout the brain, which in turn, might enhance learning, memory, and cognitive performance.

Although I still don’t believe that fasted cardio is necessarily superior to fed cardio, I suspect that fasted cardio might provide specific health benefits that cannot be attained with fed cardio such as: autophagy acceleration and upregulation of nerve growth factors that are solely influenced by ketone bodies.  Then again, I also wonder if there may be certain long-term health detriments that are unique to fasted cardio as a result of increased stress hormone production.  In sum, because there are likely health benefits that can only be attained from fasted cardio (autophagy and upregulation of ketone-specific NGFs), I usually train fasted.

(Source: https://smartdrugsmarts.com/episodes/episode-120-intermittent-fasting/)

My Fasted Cardio Regimen (Details)

Between 2015-2016, I had performed moderate-to-high intensity fasted cardio approximately once per week.  In the latter half of 2017, I had performed fasted cardio between 3 and 6 times per week – alternating between low intensity (elliptical) with long duration (45-60 minutes) and moderate-to-high intensity with shorter duration (20-35 minutes).  As of 2018, I’m performing moderate-to-high intensity fasted cardio at least 1 once per week in the form of steady-state jogging or running.

Before performing my fasted cardio, I usually will have undergone a strict fast (nothing other than water) for a minimum duration of 13 hours.  Less frequently, I’ll do fasted cardio after just a 10 hour fast, but then abstain from food intake for several hours post-fast because: I want to extend the fast window (and get potential benefits) and I’m less hungry after intense exercise.  The longest fast I’ve ever undergone before a fasted cardio session was 23+ hours – my energy level was extremely low.

  • Fast duration: ~13-20 hours
  • Intensity: Moderate-to-high (~60-85% RPE)
  • Cardio type: Running
  • Cardio duration: 20-45 minutes (or 3-6 miles)
  • Time of day: Morning or afternoon

Note: Occasionally I’ll perform lower-intensity fasted cardio on the elliptical or by going for a walk.  I’ll also sometimes perform moderate-to-high intensity cardio by biking.

What is my subjective experience with fasted cardio?

After performing fasted cardio consistently for the past few years, I’ve actually come to enjoy it significantly more than fed cardio.  I’ve compiled brief lists of subjective “benefits” and “drawbacks” associated with fasted cardio.  Understand that these benefits and drawbacks are personal and may not apply to you (perhaps as a result of things like shorter fasting windows and/or lower intensity cardio).

Subjective benefits

  • Appetite suppression: It seems as though fasted cardio is sometimes superior to fed cardio in the suppression of my appetite.
  • Enjoyment: Though I enjoy both fed and fasted cardio, I derive greater enjoyment from fasted cardio.
  • Mental clarity: I feel much better mentally when performing cardio in the fasted state – could be those ketones, eh?
  • Postponing food intake: I’ve found fasted cardio to be helpful for the postponement of food intake (which helps if I’m trying to cut weight).
  • Post-workout energy: There’s a noticeable stress response that I’ve experienced from high-intensity fasted cardio, however, this stress response isn’t overly bothersome.  In fact, the stress response seems to accelerate my thinking whereby it becomes easier to generate ideas, as well as enhance my alertness/vigilance.

Subjective drawbacks

  • Dehydration: It seems as though I’m more prone to mild dehydration when performing fasted cardio compared to fed cardio.
  • Fatigue: My energy level is significantly lower when performing high-intensity and/or long-distance running in the fasted state than in the fed state.
  • Higher RPE: It seems as though exercising in the fasted state takes more effort (at least when first starting) than exercising in the fasted state.
  • Performance: Because my energy level is lower, my performance is worse during fasted cardio than fed cardio. (I’m usually marginally slower during fasted cardio than fed cardio).
  • Stress response: Though I experience higher post-workout energy after fasted cardio than fed cardio, I suspect that this stress response could be excessive and/or damaging in certain ways (such as by excessive release of cortisol).

Do I always perform fasted cardio?

No.  On many occasions I will work for most of the morning (sometimes the entire morning), then eat a small meal (300-500 calories), work some more, and then do “fed cardio.”  I usually notice that I’m less hungry, less lightheaded, and more relaxed when performing fed cardio than fasted cardio.  I also notice that my performance is substantially better when doing fed cardio than fasted cardio – probably making for a more efficient workout (such that I have an easier time burning more calories in a shorter amount of time).

Do you prefer fasted cardio or fed cardio?

If you have a preference for fasted or fed cardio – mention which cardio-type you prefer and some reasons why (e.g. fed cardio fits better within my schedule) in the comments.  To help others better understand your cardio routine and general health, consider answering some of the questions listed below in your comment.

  • How often do you perform cardio? (e.g. 3 times per week)
  • What type of exercise do you do for cardio? (e.g. biking)
  • What is the average duration of your cardio sessions? (e.g. 45 minutes)
  • Are you underweight, normal weight, overweight, or obese?
  • Are you “out of shape” or physically fit?
  • Do you think there are specific health benefits from fasted cardio that cannot be attained from fed cardio?
  • If you perform fasted cardio, how long do you fast before your cardio sessions?
  • If you perform fasted cardio, what is the average intensity and duration of your cardio sessions?
  • For what duration have you consistently engaged in cardiovascular exercise? (e.g. 3 years)

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