Intermittent fasting and sports - benefits and negatives

What is intermittent fasting?

Intermittent fasting (IF) is a way of eating that alternates between periods of eating and periods of fasting. It is not a specific diet, but a time frame of when we eat and when we don't.
The most commonly used is time-restricted eating (TRE) – a regimen where all daily food intake occurs within a specific window, typically 6 to 10 hours a day , while the rest of the day (2–6 p.m.) is fasting. During fasting, only non-energy drinks are usually allowed, such as water, black coffee, or unsweetened tea.

Another form is alternate-day fasting (ADF) – alternating days with normal food intake and days with very low energy intake (approx. 500–800 kcal). Also known is the 5:2 regimen, where you eat normally for five days and severely restrict calories for two days.

What benefits does intermittent fasting bring?

What could be the negatives?

Intermittent fasting is generally safe, but it is not suitable for everyone .
In some people it may cause: fatigue, headaches, irritability or concentration problems , especially in the first few days. It may also be less suitable for women , in whom prolonged fasting can affect hormonal balance, and is not recommended for people prone to eating disorders . If the regime is set up incorrectly, it can also lead to excessive hunger and overeating during the allowed window, completely losing the original effect.

Does intermittent fasting affect athletic performance?

In terms of performance, there is no evidence of direct improvement . Short-term studies show that, while maintaining adequate energy and nutrient intake , there is no deterioration in performance in medium-duration sports (e.g., soccer, running up to 90 minutes). On the other hand, there is no evidence that IF leads to increased performance or better training adaptations .

For endurance and strength athletes, meal timing is key – the body needs energy before training and enough nutrients after.

Longer periods without food can limit muscle protein synthesis (MPS) and thus reduce growth or recovery potential, making IF not ideal for athletes looking to maximize performance or muscle building .

What about muscle mass?

Research confirms that intermittent fasting does not lead to greater muscle loss than other diets when dieting, as long as adequate protein intake is ensured. Conversely, for people trying to gain muscle, the results may be weaker , because the shorter eating window makes it more difficult to consume enough calories and protein. Another factor is that prolonged fasting reduces the rate of protein synthesis , which slows muscle growth.

Athletes who want to try IF should carefully plan the timing of meals relative to training and increase nutrient density (especially protein) within the allowed window.

Summary

Intermittent fasting can be an effective method for weight control and improving metabolic health , but its effects are not better than those of regular caloric restriction at the same energy deficit. In the general population, weight loss is approximately 0.25–0.35 kg per week , with alternate-day fasting up to about 0.75 kg per week .

For athletes, IF may be an interesting alternative for the off-season or for better body weight control, rather than as a strategy for performance improvement .
Every body reacts differently – while intermittent fasting may be natural and sustainable for some, it can sap energy and concentration for others.

Resources:

1. https://pubmed.ncbi.nlm.nih.gov/34633860/
2. https://pubmed.ncbi.nlm.nih.gov/35070352/
3. https://bmjopensem.bmj.com/content/8/1/e001206
4. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC9998115/
5. https://pubmed.ncbi.nlm.nih.gov/31283627/
6. https://www.healthline.com/nutrition/11-myths-fasting-and-meal-frequency?fbclid=IwAR0xYaTHgU-_848fzt6eVOpo8QYjknBqhvnMtVRkL-A2OoRsdLwEUIxsxcw#TOC_TITLE_HDR_13