Intermittent fasting has become a popular weight loss strategy in recent years, with many people believing that it is superior to traditional dieting methods. However, is this really the case?

Intermittent fasting involves blocking off long stretches of time in your diet schedule where no eating is allowed and condensing all eating into a short period, usually in the evening. Unfortunately, as we know from the nutrient timing diet principle, this diet structure is not the best option for body composition (muscle retention) or sport performance.

Adherence is one of the purported benefits of fasting. While fasting might make temporary adherence to a hypocaloric diet easier for some people, it also has downsides for a long-term healthy lifestyle. Even for adherence, the direct research on fasting does not show any significant increases in adherence compared to consuming a normal number of daily meals (3-5). Some research even shows reduced adherence to fasting protocols and less dietary continuation months after completing a fasting study compared to a calorie restriction study with normal meal spacing. This means that there are few people who see adherence benefits from fasting. For the few people who do adhere better to their calorie and macro allotments using a fasting strategy, adherence will be much more valuable than optimal meal timing, but will come at a trade-off of best body composition results.

Autophagy is another benefit that is often touted by proponents of fasting. Autophagy is the process by which your body destroys its own cells and tissues. This process is critical to health; without it body structures would wear down and begin to function poorly. Although fasting does increase autophagy, so does any caloric deficit. Your body must catabolize its own components to fuel basic survival needs when not enough food is being taken in. Hard exercise also significantly boosts autophagy. The only evidence for a special boost to autophagy from intermittent fasting has so far come from studies of starving yeast and remains unproven in humans or mammals.

Growth hormone increases and anabolism are also touted as benefits of fasting. When you fast, your growth hormone (GH) levels increase. This is because GH gives the signal for fat and carbs to be used for fuel instead of being stored in the muscle. However, when you fast, your body releases GH in order to feed on the stored contents of some of its cells. The purported anabolic effect however is not possible under fasted conditions. Growth hormone is only anabolic when there is an abundance of protein and calories. If you fast to raise growth hormone levels, the lack of excess nutrients leaves out the potential muscle growth benefits. If you give this just a bit of thought, it starts to sound absurd––you are supposed to get bigger by not eating for long periods of time. This is contradicted by the physiological need to supply muscles regularly with amino acids (and the machinery that builds muscle with calories to operate). Although GH levels are increased, the fasted state inhibits their muscle building effects and fasting itself creates a catabolic environment, making net muscle loss a more likely outcome.

In conclusion, while fasting may make temporary adherence to a hypocaloric diet easier for some people, it is not the best option for body composition (muscle retention) or sport performance. If you feel you can adhere to your diet program better with fasting, know that you will be trading off maximum muscle retention or growth depending on the diet. Some muscle tissue will inevitably be used for fuel during long periods without eating (especially on a hypocaloric diet). If you would like keep a diet structure similar to fasting while conserving a bit more muscle than traditional fasting allows, just consume meals comprised of only protein periodically during the “fasting” period and eat the rest of your macros in meals.