Intermittent Fasting (IF) is a popular dieting strategy used to lose body fat in both sedentary and active individuals. There is also research emerging investigating the impact of IF on longevity. Some athletes use this strategy in hopes of getting an edge on athletic performance. Proponents of IF argue that it enhances recovery and training adaptations, increases metabolic rate and burns more body fat. However, current research does not support many of these claims.
*Editor’s note: While there is interesting research looking into the impact of IF on circadian rhythm, disease, lifespan, that is not the focus of this current article.
What is Intermittent Fasting?
The medical definition of fasting is abstaining from food and caloric beverages for 8 to 12 hours. Intermittent Fasting is a way of eating that abstains from consuming food for a certain amount of time during the day, which is known as Time Restricted Eating. Some IF strategies also abstain from food for 24-hour periods. Popular IF strategies include 16/8 (16 hours of fast with an 8 hour eating window), 20/4 (20 hours of fast with a 4 hour eating window), 24 hour fasts twice per week, and OMAD (only one meal a day). There are a plethora of other strategies as well.
Fasting is also part of many religions, notably the Muslim practice of Ramadan, where practicioners abstain from eating and drinking during the daylight hours for one month. In fact, many studies evaluating IF look at those who participate in Ramadan
Is Intermittent Fasting Helpful for Athletic Performance?
Most of the scientific research investigating IF have evaluated male athletes who observe Ramadan or looked at performance after overnight fasts.
One study evaluating athletic performance of Muslim soccer players engaging in high intensity exercise (sprinting) found that times in a 60-minute intermittent sprint exercise were significantly slower in the fasting period than the “fed” periods, even with controlling for pre-exercise conditions such as sleep, nutrition, and training loads.(1)
Another study in 21 active males showed reduced speed, power, and vertical stiffness (a biomechanical factor that influences running economy) in the fasted state when compared to a fed state.(2)
A 2018 literature review evaluating 46 studies for their results on post-exercise metabolic adaptations in continuous aerobic (think: running) and intermittent exercise performance (think: stop-go-sports) found a statistically significant benefit to pre-exercise eating before longer duration exercise, but no benefits to pre-exercise feeding to shorter duration exercise.(3)
What About Female Athletes?
A large and important gap in the literature is looking at how IF affects females, from performance, metabolic, hormonal, and psychological perspectives.
There may be links to within-day energy deficits (spending x amount of time under-fueled compared to one’s total daily energy needs) with menstrual dysfunction in female athletes.(4) One study showed that female athletes with menstrual dysfunction spent more time in catabolic states than female athletes with normal menstruation. The results suggested “within‐day energy deficiency was associated with clinical markers of metabolic disturbances.”(4) These are important considerations as menstrual function is a marker for underfueling in age-appropriate females and a symptom of RED-S , which is when under-eating causes poor physical and psychological side effects.
Furthermore, athletes in physique sports (gymnastics, diving, figure skating) and endurance sports (running and cycling) are at higher risk of under-eating and eating disorders. It’s estimated that the prevalence of disordered eating patterns range from 6-45% of female athletes and 0-19% of male athletes.(5,6)
Given that a history of dieting and weight control methods are a risk factor for eating disorders, there should be much evidence that the benefits of a dietary strategy far outweigh the risks, especially with a dietary strategy that involves prolonged restriction of food.
Does fasting make me more fat adapted?
Fasting prior to exercise increases fat oxidation during the workout, which may be advantageous for endurance athletes.(16) Becoming more efficient at burning fat as fuel is likely helpful for endurance athletes to more efficiently access multiple energy systems during exercise. However, the overall research on fasted training for improved performance is quite mixed.
If someone is interested in fasted training to improve fat adaptation for endurance sports, a convenient strategy is to engage in easy-effort workouts first thing in the morning after an overnight fast without eating prior to the workout.(7) Don’t try this before key workouts, hard, or prolonged efforts.
Proponents claim that IF burns more body fat, but from a weight loss (and more specifically, fat loss) perspective, this only reigns true if someone is in a caloric deficit, meaning a shortage of calories consumed relative to the amount of calories needed to sustain current body weight. Intermittent Fasting will not result in weight loss in the absence of a calorie deficit. This would defy the laws of physics in relation to energy balance. Individuals who have lost weight using this dietary strategy were simply in a calorie deficit.
A recent, well-executed study evaluating IF as a weight loss strategy (where one of the authors actually used an IF dietary strategy himself, and thus had motivation to investigate it) found no statistically significant difference in fat loss and weight loss between IF and non-IF groups. The study also found no difference in health markers (like fasting blood sugar, HbA1c and blood lipids). One of the most interesting results in the study was the IF group spontaneously reduced their physical activity (number of steps) than the non-IF group.(8)
I heard Intermittent Fasting helps my body produce Growth Hormone, which will help me recover better and build more muscle. Is this true?
Despite the fact that exogenous Human Growth Hormone is a banned substance by The World Anti-Doping Agency (WADA), there is actually limited evidence that using it improves athletic performance.(15) In fact, it’s a total misnomer that Growth Hormone is anabolic for skeletal muscle in adults! Growth Hormone actually helps with whole body anabolism in the tissues and organs, not skeletal muscle.(9)
It’s true that fasting will cause large releases in Growth Hormone in our bodies. This is because it helps mobilize our stored fuel. Think of it this way: If you haven’t eaten in a long time, your body doesn’t have access to the quick fuel from recently digested food, but it still needs to make energy for various processes. So, it’s going to need to obtain energy from our stored carbs (glycogen), fat, and protein. A release in Growth Hormone is part of the process that signals our body to use the stored energy.
There is a plethora of literature to support that incorporating regular energy and protein evenly spaced throughout the day will maximize muscle growth and repair. This means that consuming food with adequate protein 3 to 4 times per day will stimulate more muscle growth than consuming the same amount of total protein 1 to 2 times per day.(10,11,12, 3)
Some of the literature looking at changes in muscle mass and resistance training with IF strategies found that the incorporation of fasting did not result in muscle losses. The absence of muscle loss is not synonymous with maximizing muscle gain! If someone is looking to build muscle, there isn’t good data to show it’s beneficial. Plus, the caloric surplus and elevated protein needs required to maximize muscle hypertrophy (growth) would be hard to accommodate with a fasting strategy since the food is eaten throughout a shorter window of time.
One small study evaluated body composition changes in eight lean men doing 20 hour fasts, every other day for two weeks. Results of the study showed no muscle breakdown. One interesting point to note is the participants were men who normally exercised less than three times per week, which is minimal exercise. The authors did address that the short duration of the study may also not have been enough time to see changes in muscle. Another interesting result from this study was a slowing in resting metabolic rate in the fasted men.(14)
While looking at longer term fasting would be more insightful to evaluate changes, most of the literature looking at body composition changes looks at short-term fasting. Most didn’t show any negative effects to muscle mass. So, short term fasting strategies might not be detrimental to muscle breakdown, but as mentioned earlier, it’s not the best strategy for maximizing muscle gain.
Bottom Line?
Based on current literature, IF will not improve athletic performance in both endurance and high intensity exercise. It will not provide enhanced recovery or maximize muscle growth. It also does not increase metabolic rate, rather multiple studies suggest it slows metabolic rate. Fasting may help with fat adaptation, which could be beneficial for endurance athletes, but literature is still unclear if it actually helps improve athletic performance. Additionally, given the high energy output of many athletes, IF could cause under-eating, which is physiologically and psychologically detrimental.
The feasibility and psychological aspect of diets should also not be overlooked and haven’t been addressed in the literature or this article. It’s important to ask yourself before starting any diet:
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Is this way of eating sustainable?
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Is this diet strategy unsettling to my relationship with food?
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Does this diet work with my lifestyle?
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Would I feel comfortable telling my children I’m on this diet, and why?
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What is my gut telling me about this diet?
works cited
- Aziz AR, Che Muhamad AM, Roslan SR, Ghulam Mohamed N, Singh R, Chia MYH. Poorer Intermittent Sprints Performance in Ramadan-Fasted Muslim Footballers despite Controlling for Pre-Exercise Dietary Intake, Sleep and Training Load. Sports (Basel). 2017;5(1):4. Published 2017 Jan 6. doi:10.3390/sports5010004
- Cherif A, Meeusen R, Farooq A, Ryu J, Fenneni MA, Nikolovski Z, Elshafie S, Chamari K, Roelands B. Three Days of Intermittent Fasting: Repeated-Sprint Performance Decreased by Vertical-Stiffness Impairment. Int J Sports Physiol Perform. 2017 Mar;12(3):287-294. doi: 10.1123/ijspp.2016-0125. Epub 2016 Aug 24. PMID: 27248138.
- 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: 1476– 1493. https://doi.org/10.1111/sms.13054
- Fahrenholtz, IL, Sjödin, A, Benardot, D, et al. Within‐day energy deficiency and reproductive function in female endurance athletes. Scand J Med Sci Sports. 2018; 28: 1139– 1146. https://doi.org/10.1111/sms.13030
- Bratland-Sanda S, Sundgot-Borgen J. Eating disorders in athletes: overview of prevalence, risk factors, and recommendations for prevention and treatment. Eur J Sport Sci. 2013;13(5):499-508
- Joubert, Lanae & Gonzalez, Gina & Larson, Abigail. (2020). Prevalence of Disordered Eating Among International Sport Lead Rock Climbers. Frontiers in Sports and Active Living. 2. 10.3389/fspor.2020.00086.
- Impey, S.G., Hearris, M.A., Hammond, K.M. et al. Fuel for the Work Required: A Theoretical Framework for Carbohydrate Periodization and the Glycogen Threshold Hypothesis. Sports Med 48, 1031–1048 (2018). https://doi.org/10.1007/s40279-018-0867-7
- Lowe DA, Wu N, Rohdin-Bibby L, et al. Effects of Time-Restricted Eating on Weight Loss and Other Metabolic Parameters in Women and Men With Overweight and Obesity: The TREAT Randomized Clinical Trial. JAMA Intern Med. 2020;180(11):1491–1499. doi:10.1001/jamainternmed.2020.4153
- Velloso CP. Regulation of muscle mass by growth hormone and IGF-I. Br J Pharmacol. 2008;154(3):557-568. doi:10.1038/bjp.2008.153
- Areta et al. (2013). Timing and distribution of protein ingestion during prolonged recovery from resistance exercise alters myofibrillar protein synthesis. The Journal of physiology, 591(9), 2319-31.
- Moore et al. (2012). Daytime pattern of post-exercise protein intake affects whole-body protein turnover in resistance-trained males. Nutrition & metabolism, 9(1), 91.
- Norton et al. (2017). Meal Distribution of Dietary Protein and Leucine Influences Long-Term Muscle Mass and Body Composition in Adult Rats. The Journal of Nutrition, 147(2), 195-201
- Schoenfeld, B.J., Aragon, A.A. How much protein can the body use in a single meal for muscle-building? Implications for daily protein distribution. J Int Soc Sports Nutr 15, 10 (2018). https://doi.org/10.1186/s12970-018-0215-1
- Maarten R Soeters, Nicolette M Lammers, Peter F Dubbelhuis, Mariëtte Ackermans, Cora F Jonkers-Schuitema, Eric Fliers, Hans P Sauerwein, Johannes M Aerts, Mireille J Serlie, Intermittent fasting does not affect whole-body glucose, lipid, or protein metabolism, The American Journal of Clinical Nutrition, Volume 90, Issue 5, November 2009, Pages 1244–1251, https://doi.org/10.3945/ajcn.2008.2732
- Liu H, Bravata DM, Olkin I, Friedlander A, Liu V, Roberts B, Bendavid E, Saynina O, Salpeter SR, Garber AM, Hoffman AR. Systematic review: the effects of growth hormone on athletic performance. Ann Intern Med. 2008 May 20;148(10):747-58. doi: 10.7326/0003-4819-148-10-200805200-00215. Epub 2008 Mar 17. PMID: 18347346.
- Impey, S.G., Hearris, M.A., Hammond, K.M. et al. Fuel for the Work Required: A Theoretical Framework for Carbohydrate Periodization and the Glycogen Threshold Hypothesis. Sports Med 48, 1031–1048 (2018). https://doi.org/10.1007/s40279-018-0867-7
