Over the past few years, not a week goes by that an athlete or client doesn’t ask me about intermittent fasting and if it’s right for them. My answer is always the same: It depends.

Fasting is certainly not a new concept; it is practiced for religious, spiritual, physical, and longevity reasons. But recent attention to different forms of time-restricted eating has increased the past few years because animal studies and, more recently, human studies, link intermittent fasting to a wide range of health benefits.

A recent review in The New England Journal of Medicine concludes that, “Pre-clinical studies and clinical trials have shown that intermittent fasting has broad-spectrum benefits for many health conditions, such as obesity, diabetes mellitus, cardiovascular disease, cancers, and neurologic disorders.”1

Defining The Term Fasting

Intermittent fasting has become an umbrella term in the public media, but to decide which, if any, form of fasting is right for you, we must make a few distinctions. In all forms, fasting takes advantage of the body’s desire to maintain homeostasis, a tightly controlled set of internal conditions optimal for survival.

At our most primitive level, we are survival machines. When our internal conditions are disrupted – in this case, depriving food and nutrients – the body sets in motion different pathways to survive the threat and return to homeostasis.

Short-term calorie restriction from fasting turns on a metabolic switch that enables us to efficiently utilize fat stores for energy while protecting lean muscle mass. Because we aren’t aware of intentionally introducing this stress on the body, it triggers additional measures to prioritize maintaining homeostasis. 

What Is Short-Term Fasting?

Most often, intermittent fasting is time-restricted eating; i.e., restricting the day's food intake to a specific period of time each day. The most common approach is the 8:16 method, where food is consumed only during an eight-hour window, with different 8-hour “windows” working better for different individuals.

Time-restricted eating can be designed to intentionally reduce calories, or to place no restrictions on the foods consumed, so long as eating falls within the intended fueling period.

Short-term fasts are associated with beneficial impacts on glycemic control, insulin levels, and weight management in humans, as well as supporting cardiovascular health and cognitive performance in animal studies.2-6 In this article, the term intermittent fasting refers to time-restricted eating. 

What Is Prolonged Fasting?

Another form of fasting – prolonged fasting – includes alternate day fasting and the 5:2 method, which involve fasts lasting between 24 and 36 hours. These fasts involve eating 600 or fewer calories either every other day (alternate day fasting) or spacing two separate fast days with five fueling days each week (5:2 method).

Research suggests some benefits are only triggered with longer fasts; however, these fasts are harder to plan and consistently implement. These fasts include more severe calorie restriction and often include the body depleting its glycogen stores and switching to burning fat and ketones for energy.

The focus of prolonged fasting centers on the biologic process called autophagy, a survival process triggered by starvation. Autophagy, from the Latin for “self-eating,” involves the body breaking down damaged or dysfunctional cells and recycling them into nutrients to feed a body that isn’t eating.

Research is currently underway to better understand how autophagy impacts longevity and health; however, extended fasting longer than 36-48 hours should involve medical supervision.

Circadian Rhythm and Fasting

The benefits of short-term fasts and time-restricted eating focuses on the body’s 24-hour clock, or circadian rhythm, that controls the body’s sleep/wake cycle, as well as many metabolic and hormone cycles throughout the day. The most well-known factor that regulates the circadian rhythm is the rise and fall of the sun; however, a fast-paced, high-energy life can disrupt healthy biorhythms.

An increase in screen time from cell phones and laptops, growing access to calorie-dense foods, and a decrease in physical activity can all have negative impacts on the circadian rhythm leading to metabolic and sleep disturbances which can feel similar to jet lag

As humans evolved, our metabolism and energy systems grew to take advantage of the circadian rhythm, making us more efficient at fueling physical activity during awake hours when hunting and gathering occurred, while promoting energy conservation, recovery, and repair during the hours of low activity and sleep. 

As current lifestyles have allowed us to be more active and stimulated throughout the day, and thus shifting the normal biorhythms from the traditional sleep/wake cycle, we see negative impacts on health and body composition.7-9

It is thought that decreasing the eating period promotes a return to traditional circadian rhythms and minimizes the negative impacts of a disrupted 24-hour internal clock. Each organ system in our body has its own internal clock, controlling everything from how we metabolize food and detoxify our bodies to how fast our hair and nails grow.

Research has linked time-restricted eating to improvements in normal response to inflammation, glycemic control, cardiac, obesity, and even aerobic performance.7-10

When we eat, the body releases the hormone insulin, which converts food into glucose, which is converted to energy or stored as fat. When we fast, insulin levels drop, signaling our body to break down fat stores for energy. 

A recent study showed that when the eating period is decreased to eight hours, participants saw a 3-percent reduction in total weight, as well as a 4-percent reduction in abdominal fat.1

Although it is thought that some of these improvements are tied to the increased breakdown in fat stores for energy in the absence of food, researchers also found that, without any instructions to do so, participants consumed 8-9 percent fewer calories when the feeding period shrunk to eight hours.1

Interestingly, the benefits of fasting appear even when fewer calories are not consumed. When asked to consume an equal amount of calories in a 6-hour period as opposed to a 12-hour period, the short-term group exhibited decreased appetite and increased fat metabolism compared to the conventional group.11

An analysis of men considered at risk for developing type 2 diabetes found that when eating was limited to a 9-hour feeding period, both blood sugar control and triglyceride levels were improved. 

In obese and overweight subjects, time-restricted eating periods of 6-9 hours are associated with improved blood pressure control, food cravings, mental focus, and cognitive performance.12-15

Intermittent Fasting and Athletes

A recent review from the American College of Sports Medicine (ACSM) explored the results of combining exercise and time-restricted eating. An 8-week study of strength-trained males where calorie intake was standardized showed that while both time-restricted (nine hours) and unlimited eating groups increased muscle mass, only the time-restricted group had decreases in total body mass and fat mass, with improvements in glucose, cholesterol, and triglyceride levels.16 

A follow-up study in female strength-trained subjects reported that females who consumed their calories in an 8-hour period had no loss of muscle strength or muscle growth compared to the unrestricted-eating control group.17 Other studies combining time-restricted eating and exercise have found similar improvements in body composition and lipid levels, although none have found positive or negative effects on performance.  

Very recent research is looking at differences in the timing of the restricted-eating period, and a theory is emerging that a new eating period is more beneficial to individuals combining intermittent fasting with exercise.

When subjects restricted eating to 8 a.m. to 2 p.m., they reported less hunger and a decrease in ghrelin (the hormone responsible for stimulating appetite), had lower glucose levels throughout the whole day, and lower levels of the stress hormone cortisol in the evening compared to their unrestricted meal-time counterparts.11,18 

By impacting both feelings of hunger and hormonal changes, the early time-restricted periods appear to be the most promising for those training. The ACSM review concludes that, “when individuals undertake regular physical activity, complementing exercise training with time-restricted eating may help modify behavioral patterns of eating, (i.e., reduced end-of-day snacking/alcohol consumption) and enhance overall health benefits.”10

Finally, it should be noted that all studies showing muscle maintenance and growth in time-restricted eating provided an adequate amount of protein and total calories. Sustained intakes of nutrients below daily needs and fasting periods greater than 24-36 hours require more attention to detail and input of a sports medicine team to support athletes at the highest levels of sport.

The current recommendation for pairing exercise with intermittent fasting would pair exercise timing (either during the feeding period or just before) around the feeding period to maximize the opportunity to replace the nutrients lost during exercise and provide the body the nutrients needed to promote muscle growth, repair, and recovery.


  1. de Cabo R, Mattson M. Effects of intermittent fasting on health, aging, and disease. N Engl J Med 2019;381(26):2541-2551. 
  2. Gabel K, Hoddy K, Haggerty N, et al. Effects of 8-hour time restricted feeding on body weight and metabolic disease risk factors in obese adults: A pilot study. Nutr Healthy Aging 2018;4(4):345-353.
  3. Mattson M, Moehl K, Ghena N, et al. Intermittent metabolic switching, neuroplasticity and brain health. Nat Rev Neurosci 2018;19(2):63-80.
  4. Mattson M, Longo V, Harvie M. Impact of intermittent fasting on health and disease processes. Ageing Res Rev 2017;39:46-58.
  5. Rangan P, Choi I, Wei M, et al. Fasting-mimicking diet modulates microbiota and promotes intestinal regeneration to reduce inflammatory bowel disease pathology. Cell Rep 2019;26(10):2704-2719.e6.
  6. Rynders C, Thomas E, Zaman A, et al. Effectiveness of intermittent fasting and time-restricted feeding compared to continuous energy restriction for weight loss. Nutrients 2019;11(10). doi:10.3390/nu11102442
  7. Asher G, Sassone-Corsi P. Time for food: the intimate interplay between nutrition, metabolism, and the circadian clock. Cell 2015;161(1):84-92.
  8. Panda S. Circadian physiology of metabolism. Science 2016;354(6315):1008-1015. 
  9. Gabriel B, Zierath J. Circadian rhythms and exercise – re-setting the clock in metabolic disease. Nat Rev Endocrinol 2019:1. 
  10. Parr E, Heilbronn L, Hawley J. A time to eat and a time to exercise. Exerc Sport Sci Rev 2020;48(1):4-10.
  11. Ravussin E, Beyl R, Poggiogalle E, et al. Early time‚Äźrestricted feeding reduces appetite and increases fat oxidation but does not affect energy expenditure in humans. Obesity 2019;27(8):1244-1254. 
  12. Sutton E, Beyl R, Early K, et al. Early time-restricted feeding improves insulin sensitivity, blood pressure, and oxidative stress even without weight loss in men with pre-diabetes. Cell Metab 2018;27(6):1212-1221.e3.
  13. Wilkinson M, Manoogian E, Zadourian A, et al. Ten-hour time-restricted eating reduces weight, blood pressure, and atherogenic lipids in patients with metabolic syndrome. Cell Metab December 2019. doi:10.1016/j.cmet.2019.11.004
  14. Mattson M, Wan R. Beneficial effects of intermittent fasting and caloric restriction on the cardiovascular and cerebrovascular systems. J Nutr Biochem 2005;16(3):129-137.
  15. Witte A, Fobker M, Gellner R, et al. Caloric restriction improves memory in elderly humans. Proc Natl Acad Sci U S A 2009;106(4):1255-1260.
  16. Moro T, Tinsley G, Bianco A, et al. Effects of eight weeks of time-restricted feeding (16/8) on basal metabolism, maximal strength, body composition, inflammation, and cardiovascular risk factors in resistance-trained males. J Transl Med 2016;14(1):290.
  17. Tinsley G, Moore M, Graybeal A, et al. Time-restricted feeding plus resistance training in active females: a randomized trial. Am J Clin Nutr 2019;110(3):628-640.
  18. Jamshed H, Beyl R, Della Manna D, et al. Early time-restricted feeding improves 24-hour glucose levels and affects markers of the circadian clock, aging, and autophagy in humans. Nutrients 2019;11(6). doi:10.3390/nu11061234