What Science Says about Fasted Exercise

Brady Holmer
8 min readJan 7, 2021

There’s probably no more trendy topic in health and nutrition right now than fasting. Intermittent fasting (IF), while an ancient practice, has gained newfound popularity for the supposed benefits it could have for weight loss, metabolic health, and even longevity.

An area where fasting has also gained attention is in relation to exercise. Many health gurus and performance “experts” advocate fasted exercise. This means undergoing your (typically morning) workout after fasting overnight. In my opinion, fasted exercise can have a place in most training regimens, if used strategically.

When you’re fasting, blood glucose, glycogen, and insulin decrease, and our body upregulates enzymes and pathways that oxidize fatty acids to produce energy. I.e. — we are in a “fat burning” state. This is why some intermittent fasting and time-restricted feeding (TRF) studies find that this dietary regimen may be good for weight loss (though not all find this benefit).

Source: MySportScience.com

Regarding fasted exercise, the theory is that working out while fasted should enhance your ability to burn fat and increase the actual amount of fat you burn — leading to metabolic adaptations and an improved body composition; more lean mass and less fat mass. There are also other potential benefits of fasted exercise including improved glucose tolerance and insulin sensitivity, increased mitochondrial adaptations, and exercise-induced ketone production.

The caveat here is that, as anyone who works out knows, food is a source of energy, and a light pre-workout snack is often going to be beneficial for overall mood and performance. So fasted exercise is a “tradeoff” of sorts.

This is why I say that a “strategic” use of fasted exercise could be beneficial. Lower intensity training sessions (an easy run lasting 60 minutes or less) could be done in the fasted state, since the energy needed for this type of effort and duration could be provided through aerobic metabolism via fat oxidation. More high intensity or prolonged sessions might benefit from a bit of carbohydrate pre workout.

Unfortunately, there hasn’t been much research on whether exercising while fasted is actually superior when it comes to health outcomes. I want to note here that there is unequivocal evidence (published and anecdotal) that fasted exercise is NOT better for actual exercise performance. In no way will you run faster or lift heavier in the fasted vs. fed state. This is well known. What we are talking about here is whether certain beneficial adaptations might occur during fasted exercise that, in the long term, will benefit health.

There are several different realms where fasted exercise has been investigated including body composition, performance outcomes, and what we will generally refer to as metabolic/mitochondrial adaptations.

Body composition

A study conducted in 20 healthy female participants found that fasted exercise was not any better than fed exercise for reducing body weight and total fat mass during a diet designed for weight loss (a hypocaloric diet).

The training in this study was moderate-intensity aerobic exercise for 1 hour/day, 3x/week. The fasting group performed training sessions after an overnight fast, and the fed group was given a pre-workout nutrition shake that contained 40g of carbohydrates, 20g of protein, and 0.5g of fat. The fasted group consumed the shake immediately after each training session.

Schoenfeld 2014

As you can see in the table above, both groups lost weight and fat mass during the study, but there were no differences between fed and fasted groups for the amount of weight and fat lost during the study. I.e. fed and fasted exercise are equally effective.

Another study compared the effects of fasted vs. fed high-intensity interval training (HIIT) for six week on body weight, fat mass and body fat %, and lean mass in overweight women. They performed 3 sessions/week of HIIT for 6 weeks. The fasted group completed all sessions after an overnight fast, and the fed group consumed a pre-exercise meal of an energy bar, yogurt, and orange juice (the fasted group consumed this same meal post-exercise).

Again — differences between groups were non-existent. Both groups had reduced body fat % and increased lean body mass after the training, but no superior effects of fasted exercise were observed.

Gillen 2013

Performance

As I mentioned earlier, there is hardly a need to debate whether fasted exercise actually improves exercise performance. Exogenous carbohydrates and fat are major fuel sources during exercise, and providing them via pre-workout feeding will, in most cases, enhance performance.

To summarize performance outcomes, we need to look no further than a meta-analysis published in 2018 that evaluated all of the studies comparing fed vs. fasted exercise on outcomes of exercise performance. These weren’t training studies, but rather studies that measured acute exercise performance.

Here are some of the highlights:

Aerobic exercise >60 minutes in duration

  • 54% of studies included in the review found that pre-exercise feeding enhanced longer-duration aerobic exercise performance
  • The remaining studies found no difference between fed vs. fasted exercise

Aerobic exercise <60 minutes in duration

  • 57% of studies found no difference between fed vs. fasted exercise on performance
  • The remaining studies found that pre-exercise feeding enhanced short-duration aerobic exercise performance

Anaerobic exercise

  • Only 4 studies included, but all found that fed exercise either enhanced performance or observed no differences between fasted vs. fed exercise on anaerobic exercise performance

Take home message: fasted exercise NEVER improves performance

Metabolism/Mitochondrial adaptations

Perhaps one of the most well-supported theories on the benefits of fasted exercise is its ability to increase your ability to oxidize or burn fat. This happens, because, as I stated earlier, exercising with reduced blood glucose/glycogen means you’ll upregulate your fat-burning enzymes and pathways acutely which, over time, can lead to gene upregulation for these beta-oxidation enzymes and also help to improve your mitochondrial capacity to oxidize fat and produce cellular ATP.

Increased fat oxidation during fasted exercise could also have beneficial metabolic adaptations including a better ability to regulate glucose levels and improved insulin sensitivity.

A study in healthy males found that, during a high-calorie, high-fat diet, fasted aerobic exercise (4 days/week for 6 weeks) prevented weight gain, improved glucose tolerance and insulin sensitivity, and elevated levels of glucose-transport proteins and mitochondrial enzyme mRNA levels more than a group who completed exercise in the fed state.

Van Proeyen 2010

However, no differences in mitochondrial adaptations between fed and fasted exercise were found in a HIIT study (cited earlier; Gibala 2013), nor were any improvements in insulin sensitivity observed after fasted exercise training OR fed exercise training.

Back to the meta-analysis. It contains very few studies on the effects of long-term fasted vs. fed training on metabolic adaptations (because very few studies exist). Most studies examined the acute (short-term) effects of fasted exercise.

Overall, some key points regarding fed vs. fasted exercise were noted:

  • Fasted exercise leads to increased post-exercise AMPK and SIRT1 protein expression — these are both implicated in mitochondrial biogenesis and other beneficial metabolic adaptations
  • Circulating free-fatty acids are increased after fasted exercise (duh!)

Take home message: fasted exercise likely increases markers of mitochondrial biogenesis in the short term, but training studies need to be conducted to determine the long-term effects.

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The lack of longer-term training studies on fasted exercise clearly signals the need for more research in this area. Acute studies provide valuable information, but to really determine the impact of fasted exercise on health, we need studies lasting 8–12 weeks in duration (or more). Some observational studies on people who exercise fasted vs. fed would also provide insight into the long-term health benefits of this practice.

What we can conclude from the literature is that fasted exercise may be superior for improving body composition, increasing mitochondrial biogenesis, and increasing glucose tolerance and insulin sensitivity.

However, these effects don’t seem to be consistent among studies.

I think another unexplored area is to look at how fasted exercise may benefit markers of health related to longevity and aging. For instance — how does fasted exercise affect ketone production, brain health, or longevity-related genes? If exercising fasted DOES improve glucose tolerance and insulin sensitivity, then it may also have beneficial impacts for longevity and healthspan; as glucose and insulin are both known to regulate the aging process.

I would also be interested in how fasted exercise might influence cardiovascular health — vascular function, cardiac structure, blood pressure, etc.

A final thought to discuss here is one that is less scientific and more anecdotal. When it comes to exercise, I’m all about following the research. However, a big part of all of it comes down to personal preference — what works for YOU.

I’ve found that fasted exercise seems to be more enjoyable and more comfortable. I don’t find that eating before I workout makes much of a difference either way — maybe I take a slight performance hit, but overall I don’t notice a difference. In this case, the tradeoff between spending time eating and digesting something before a workout or just having some coffee and heading out the door is one I’m willing to make.

In some cases, I’ll have an energy bar or a banana, both of which sit fine in my stomach. However, I ALWAYS eat before races.

So — if you like oatmeal before a workout, have some oatmeal. If steaming coffee is the only pick-me-up you need before the morning grind, then by all means stick with it. Evidence shows that fasted exercise could have some potent health benefits when done correctly, but the most important part of it all is the exercise — whether on a full or empty stomach.

References cited

1. Aird TP, Davies RW, Carson BP. Effects of fasted vs fed-state exercise on performance and post-exercise metabolism: A systematic review and meta-analysis. Scand J Med Sci Sports. 2018;28(5):1476–1493. doi:10.1111/sms.13054

2. Van Proeyen K, Szlufcik K, Nielens H, et al. Training in the fasted state improves glucose tolerance during fat-rich diet: Fasted training and fat-rich diet. The Journal of Physiology. 2010;588(21):4289–4302. doi:10.1113/jphysiol.2010.196493

3. Schoenfeld BJ, Aragon AA, Wilborn CD, Krieger JW, Sonmez GT. Body composition changes associated with fasted versus non-fasted aerobic exercise. J Int Soc Sports Nutr. 2014;11(1):54. doi:10.1186/s12970–014–0054–7

4. Gillen JB, Percival ME, Ludzki A, Tarnopolsky MA, Gibala MJ. Interval training in the fed or fasted state improves body composition and muscle oxidative capacity in overweight women. Obesity. 2013;21(11):2249–2255. doi:https://doi.org/10.1002/oby.20379

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Brady Holmer

PhD candidate at the University of Florida — Science writing with a particular focus on exercise and nutrition interventions, aging, health, and disease.