The Scientific Evidence Surrounding Intermittent Fasting

The following article was written by Amber Simmons, PhD

Purpose of Intermittent Fasting

Intermittent fasting is not a diet, but a diet schedule that is purported to accelerate fat loss and muscle growth compared to traditional eating schedules. It is promoted primarily in the scientific community, however, there are currently zero scientific studies (as of February 2014) that have supported intermittent fasting for gaining muscle while losing fat.

With caloric restriction, intermittent fasting can lead to weight loss. In a recent review and one recent randomized clinical trial, authors conclude that intermittent fasting and daily caloric restriction are equally effective at promoting weight loss in overweight and obese individuals. However, no studies to date have been performed with athletes who require maintenance of muscle size, strength, and function.

Intermittent Fasting Approaches and Scientific Support Associated with Each
There have been several proposed protocols for intermittent fasting, from skipping one meal per day to eating only every other day. Most of these diets are promoted through webpages, blogs, and books published by exercise and diet enthusiasts. The procedures and philosophies are briefly summarized here; see the “More information” for links to advocates’ websites.

To date, despite the focused marketing of intermittent fasting to the athletic community, there are few well-controlled, scientific studies investigating the effects of intermittent fasting on the body composition and performance in athletes. Currently, the bulk of the scientific evidence for the health benefits of intermittent fasting has come from animal studies and the negative effects of intermittent fasting have stemmed from Muslim athletes during Ramadan, both with limited ability to be translated to the general athletic community. Although more and more human studies are being conducted to validate claims found in animals, many studies are with patients with a certain illness or condition (ex. rheumatoid arthritis, hypertension, obesity) and not in healthy, active individuals.

Similarities Among Approaches

The various intermittent fasting approaches tend to emphasize their differences (and therefore purported superiority) however, there are also many similarities. One of the key advantages of this extreme form of caloric control is that it allows people to re-conceptualize hunger. Instead of linking “hunger” with “panic” or even “desire”, “hunger” can theoretically be newly associated with “success” or “pride”, or simply ignored.

Indeed, with any method, there is a critical transition period of about 3-6 weeks during which the body and brain adapt to the new eating schedule. This period can be very uncomfortable, as restricted eating has been anecdotally associated with extreme hunger, irritability, loss of strength, loss of libido, and other negative side effects. Once the body is accustomed, however, the hunger levels may decrease and mood could become more positive compared to before the fasting program started. Elevated mood and decreased hunger on caloric restrictive diets have been noted in some but not all studies.

Intermittent fasting is not a weight loss program per se; only if calories are restricted will a person lose weight. Although intermittent fasting is one way to restrict intake of total calories to achieve weight loss , there have not been any studies to date on athletes who prioritize maintenance of muscle size and strength. In fact, there are conflicting views on whether intermittent caloric restriction vs. daily caloric restriction best preserve lean muscle mass.

All of these approaches emphasize the importance of the nutritional quality of the meals that are consumed. Nutrients such as protein, fat, fiber, vitamins, and minerals are essential for good health and, since nutrients are not consumed while fasting, they are especially important when breaking the fast. In addition, drinking a lot of water is encouraged both to stay hydrated and to alleviate hunger. John Berardi of Precision Nutrition allows green powders, green tea, and branched chain amino acids during his fast, but it is unknown how these supplements affect appetite, energy levels, muscle synthesis/breakdown, or the overall benefits of intermittent fasting.

The Periodic Fast

The Periodic Fast is a fast for 24 h and is described in an article by Dr. John Berardi. The fast can be starting at any time of the day and can be done at various frequencies, though usually not more than 1-2 times per week. This diet is also advocated by Brad Pilon (“Eat Stop Eat”), where he recommends a 24-h fast every 3-5 days for weight loss.


The LeanGains method is a daily fast promoted by Martin Berkhan, a Swedish personal trainer and nutrition writer. In his method, fasting occurs for 16 h (ex. 10pm- 2pm), then food consumption occurs in about 3 meals in an 8-h window every day. If a workout is performed that day, it is performed immediately before the first meal to ensure a large, high-protein post-workout meal. About 5-15 min before the workout, Berkhan recommends taking about 10 g branched chain amino acids (BCAAs) to halt muscle protein breakdown during your workout (those this practice is not scientifically supported). This program also recommends eating carbohydrates or fat as components of your evening meals. On exercise days, meals should include protein, vegetables, and carbohydrates; on non-exercise days, meals should include protein, vegetables, and fat.

Anecdotally, males and females may respond differently to the LeanGains diet; females may respond better to a larger eating window, for example 14 h of fasting rather than 16 h.

Theoretically, an advantage to the LeanGains approach could be that because the schedule is followed daily, the body may more accurately learn when to expect food. For example, if you always eat at 3:00 pm, 6:00 pm, and 9:00 pm, appetite hormones and metabolism may acclimate accordingly. However, this has not yet been scientifically evaluated.

The Warrior Diet

This diet schedule is another variation of the daily fast and is one step more extreme than the LeanGains diet. The Warrior Diet promotes a single, healthy meal per day (typically dinner). It claims that this pattern of eating is in sync with humans’ circadian rhythm and will promote general health while “removing harmful toxins from the body”. To our knowledge, there is no scientific evidence supporting these claims.

In a study by Stote et al., participants of normal weight consumed adequate energy to maintain body weight in one meal per day or 3 meals per day for 8 weeks. Despite consumption of the same number of calories, participants lost weight during the 1 meal per day period vs. the 3 meal per day period. In fact, fat mass was significantly reduced (p < 0.001) and lean body mass tended to be greater (p = 0.06) after 8 weeks of 1 meal per day. However, hunger steadily increased during the 8-week study period with only 1 meal per day, suggesting that appetite hormones did not acclimate. Further research is needed to both confirm these findings as well as assess changes in body composition in athletes.

Alternate Day Fasting

Alternate day fasting is when food is consumed for 24 h, then restricted for 24 h (water is available at all times) for every 2-day cycle. This is the most frequently used protocol for intermittent fasting studies with mice and rats, though few studies of this nature have been done in humans. Many animal studies looked primarily at the effects of intermittent fasting on lifespan, with varying results depending on the breed, the age of the animal when fasting was initiated, if the animals were exercised, and other factors.

Heilbronn et al. performed a study in which eight males and eight females of a healthy body weight fasted every other day for 21 days. Participants lost about 2.5 ± 0.5% of their body weight including 4 ± 1% of fat mass over the course of the 21 days. Neither fasting blood glucose nor ghrelin (an appetite hormone) concentrations changed before vs. after the intervention, but fasting insulin concentrations decreased suggesting greater insulin sensitivity. They did not observe changes in genes involved in mitochondrial biogenesis, fatty acid transport, or fatty acid oxidation suggesting that the metabolic machinery required for generating energy from fat was sufficient at the start of the study. Animal studies have routinely showed that intermittent fasting strengthens the body’s innate response to stress, and this was the first study to corroborate these findings in humans.

Other Variations

There are many variations of intermittent fasting regimens since it is not yet known which (if any) fasting protocols are best for the desired outcome.

Halberg et al. asked healthy, normal-weight participants to fast for 20 h every other day for 15 days. They observed an increase in insulin sensitivity after the fasting period as well as changes in fat metabolism. No effects were seen in regard to body weight, inflammatory cytokines, or changes in markers that are noted with an exercise intervention. On the other hand, Soeters et al. asked participants to follow the same protocol as in the study by Halberg et al. They observed no changes in insulin sensitivity, glucose uptake, or lipolysis. They also observed a decrease in mTOR phosphorylation, which is commonly associated with reduced muscle protein synthesis (MPS), although MPS was not measured in this study, nor were the participants involved in exercise.


As you can see, all diet schedules share a common theme of compartmentalizing “fasting” and “eating” periods. So many variations of these diets exist because there is no one established method that is best. Further, some individuals who try intermittent fasting use a hybrid of existing approaches to find a successful technique.

Importantly, intermittent fasting is not recommended for pregnant women, women who are breastfeeding, people with diabetes, or other people who need to closely regulate their blood sugar. In addition, there has not been research on participants who are underweight, very old, or very young (<18 yrs. old) and these populations could be at higher risk for experiencing negative consequences of fasting.

Exercising While In a Fasted State

Fasting (all approaches) can be detrimental to athletic gains for several reasons. First, meals in close proximity to your workout are essential for optimal performance, recovery, and muscle gain. Second, increased hunger sensations may hinder compliance as well as increase the potential to over-consume food when it becomes available. Despite the common belief that you will burn more fat if you exercise while fasted, doing aerobic exercise in the fasted state is not recommended. In fact:

  • Performing aerobic exercise after consuming carbohydrates does not hinder fat oxidation (Febbraio et al., 2000; de Bock et al., 2008)
  • Performing aerobic exercise fasted will also encourage loss of lean muscle mass, since muscle will be burned for fuel (Lemon and Mullin, 1980)
  • Exercising in a fasted state often does not lead to an optimal workout. In contrast, having readily available energy will allow optimal performance which will burn more calories overall and lead to the highest gains (Loy et al., 1986; Schabort et al., 1999)
  • Exercising in the fasted state vs. fed state decreases static and dynamic balance and can increase the risk of injury (Johnson and Leck, 2010)

There are fewer studies investing the effects of performing resistance training in the fasted vs. fed state, but it is expected that the same points hold true.

Intermittent fasting advocates recommend consuming at least 5 g BCAAs before a workout if exercising during your fasting period. This bolus of BCAAs in your blood stream theoretically could help preserve muscle protein during the workout, but there is no scientific evidence substantiating this claim. In one study, a BCAA infusion before a workout in the fasted state did not improve performance in one group of individuals on a graded incremental exercise test. Advocates also advise to schedule your workout/fasting schedule so that you can enjoy a complete post-workout meal, but they tend to disregard the importance for pre-workout nutrition.

Lessons From Studying Muslim Athletes During Ramadan

Because intermittent fasting is a new concept, most of the research done to date on fasting practices in athletes has originated from studying Muslim athletes during Ramadan. Muslim athletes who observe traditional Ramadan practices do not eat or drink between sunrise and sunset for 29-30 days. A 2009 review on athletic performance during Ramadan concluded that hunger and thirst cause stress to the athlete, though only modest declines in performance are typically observed. It is theoretically possible to schedule sleep and practice schedules to maintain peak performance during this time; however, in practice, it is more difficult.

A limitation to comparing Ramadan Intermittent Fasting with other intermittent fasting practices is that Ramadan introduces other changes into the athletes’ lives that affect the ability to perform optimally and achieve/maintain a strong, lean body composition. These differences include:

  • Athletes have a high propensity for dehydration during Ramadan while not during other approaches to intermittent fasting
  • In order to eat before dawn and after dusk, sleep schedules during Ramadan are sometimes altered. This can lead to a disruption in circadian rhythm and/or sleep deprivation, both of which can cause fatigue, physical stress, and a decline in performance
  • Diets during Ramadan tend to be higher in fat, sugar, and animal protein compared to diets during times other than Ramadan

Take Home Messages

  • Intermittent fasting is a dietary approach heavily promoted to the athletic community to achieve and maintain a very lean, strong physique. However, there is currently no scientific evidence supporting these claims
  • In the scientific literature, intermittent fasting with caloric restriction often yields equivalent benefits as traditional low-calorie diets in regard to changes in fat mass, alleviating discomfort due to low energy, improving insulin sensitivity, and improving blood lipid profiles
  • The most effective weight loss plans and healthy life styles are ones that can be maintained, and these habits are not the same for everyone. If intermittent fasting is appealing, give it full effort for a few weeks before evaluating success or failure of that approach
  • There are numerous webpages promoting different diet plans online (see “More information”). These are good places to get information about what has worked for other people and to get community support

More Information (Not Necessarily Based on Science):

References From Scientific Literature:

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  • Chaouachi, A, J B Leiper, N Souissi, A J Coutts, and K Chamari. 2009. “Effects of Ramadan Intermittent Fasting on Sports Performance and Training: A Review.” International Journal of Sports Physiology and Performance 4 (4) (December): 419–434.
  • De Bock, K, W Derave, B O Eijnde, M K Hesselink, E Koninckx, A J Rose, P Schrauwen, A Bonen, E A Richter, and P Hespel. 2008. “Effect of Training in the Fasted State on Metabolic Responses during Exercise with Carbohydrate Intake.” Journal of Applied Physiology 104: 1045–1055. doi:10.1152/japplphysiol.01195.2007.
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  • Johnson, S, and K Leck. 2010. “The Effects of Dietary Fasting on Physical Balance among Healthy Young Women.” Nutrition Journal 9 (January): 18. doi:10.1186/1475-2891-9-18.
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  • Lemon, P W, and J P Mullin. 1980. “Effect of Initial Muscle Glycogen Levels on Protein Catabolism during Exercise.” Journal of Applied Physiology 48 (4) (April): 624–629.
  • Longo, V D, and M P Mattson. 2014. “Fasting: Molecular Mechanisms and Clinical Applications.” Cell Metabolism 19 (2) (January): 181–192. doi:10.1016/j.cmet.2013.12.008.
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  • Soeters, M R, N M Lammers, P F Dubbelhuis, T Ackermans, C F Jonkers-Schuitema, E Fliers, H P Sauerwein, J M Aerts, and M J Serlie. 2009. “Intermittent Fasting Does Not Affect Whole-Body Glucose, Lipid, or Protein Metabolism.” American Journal of Clinical Nutrition 2009 (90): 1244–1251. doi:10.3945/ajcn.2008.27327.1.
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