The Influence of Intermittent Fasting on Muscle Mass

Intermittent fasting is a term that encompasses several nutritional strategies based on refraining from consuming meals for a set period of time. The following types can be distinguished:

which means completely refraining from ingesting food for a duration of 12 days and consuming unlimited quantities of sustenance in the remaining days; the second possibility is limiting meal intake on fasting days.

Alternate day fasting (ADF) entails maintaining calorific restrictions (approximately 25% of energy requirements) or complete abstinence from consuming meals every other day; on the remaining days, meals should be eaten as desired;

Time-restricted eating (TRE) during the nutritional window in which food consumption is allowed, usually lasts between 4-8 hours per day, however, this is an individual matter and should be tailored to your lifestyle and preferences; it is recommended to limit the number of meals during this time to 1-3.

Each type of intermittent fasting affects the availability of insulin and amino acids to some extent. Amino acids play a key role in accelerating the muscle protein synthesis process, whereas insulin prevents protein degradation in the body. Short-term fasting for only two days can lead to anabolic resistance, i.e., reduced ability of muscles to synthesize muscle proteins in response to reduced physical and fatty acids. It is also worth noting that excessive protein intake in a short period (as in the case of IF) leads to the oxidation of amino acids to ensure that the body has enough energy. Consuming meals in feeding windows results in protein being used for purposes other than muscle building. In 2021, a study was conducted comparing two nutritional models. The first one was based on consuming 3 meals in a 6-hour feeding window, while the second one in a 12-hour window. The protein content in each meal was about 0.3 g/kg of body weight. After analyzing the results of the study, it was noticed that oxidation was higher in the first group than in the second group – about 13 g more amino acids were oxidized (E. Williamson, D.R. Moore 2021). Considering the current state of knowledge, it is unlikely that the human body will develop beneficial adaptations over time, as it is not able to maximize protein synthesis at the moment of suboptimal protein intake. The IF diet may also contribute to a decrease in spontaneous physical activity (NEAT) during the day, which, in turn, may lead to the occurrence of anabolic resistance.

Intermittent fasting is a dietary approach based on a limited period of fasting. This can include full-day fasts, fasting every other day, and feeding windows. Current scientific evidence and research suggest that intermittent fasting may have a negative impact on protein metabolism in the human body. This variation is not as significant as the overall amount of protein intake, but it can have an important role in the long term. Intermittent fasting is typically used for weight loss by restricting the time for food consumption to a specific feeding window – the shorter the window, the less time for food intake. However, it is important to note that consuming large amounts of food in a short period can lead to digestive issues. When selecting a dietary approach for older and less active individuals, it is crucial to consider that there is a high risk of anabolic resistance in these cases. Therefore, it is worth considering the benefits and potential risks before deciding on a diet based on intermittent fasting.