We explore all the pros and cons of intermittent fasting and sports performance.
If you follow the latest nutrition trends, you have probably heard about intermittent fasting and its supposed superpowers. But let us focus on what matters. Is it really that effective for people looking to improve their sports performance?
What Is Intermittent Fasting?
Before diving in, let us agree on the terminology. Intermittent fasting is essentially just that: intermittent. It does not mean eating on and off, but alternating periods of fasting with periods of eating.
Contrary to popular belief, its main focus is when food is eaten, rather than how much is consumed.
It seems to have become a very popular trend in health and sport. But what benefits might it offer? Is it really safe for athletes?
Benefits of Intermittent Fasting
Some of the supposed benefits of intermittent fasting are:
- Improving insulin sensitivity and blood pressure.
- Reducing insulin resistance.
- Limiting cancer cell growth.
- Reducing hunger, making calorie control easier.
- Supporting body fat loss.
As we have seen, most benefits focus on improving health in people who need it. In sport, however, the effects are not entirely clear: while some people experience improvements in body composition (and consequently performance), the evidence suggests that fasting itself worsens performance, especially in high-level athletes. We will discuss this in more detail below.
For context, let us first discuss the types of intermittent fasting.
What Are the Main Types of Intermittent Fasting?
As mentioned, fasting has become a popular trend. There are various ways to practise it, each with its own characteristics. Below are the most popular.
- 12/12 fasting: This is something we all do: we call it sleeping. In a sense, it is what we should all be doing if we leave an appropriate gap between dinner and breakfast.
- 16/8 fasting: The most popular and accessible approach. It involves 16 hours of fasting and an 8-hour eating window.
- 24-hour fasting: Fasting for 24 hours once or twice a week.
What Are the Advantages of Intermittent Fasting for Sports Performance?
Beyond being a trend, intermittent fasting has shown potential for amateur athletes, particularly those looking to improve insulin resistance and doing low-intensity training.
By lowering insulin levels, the body may become more efficient at using fat stores for energy. This can help those aiming to optimise body composition, as it may help create a calorie deficit. In ultra-endurance events, intermittent fasting has also been suggested to improve the body’s ability to use fat as fuel during very long activities, although more research is still needed in this context.
Drawbacks of Fasting for Sports Performance
Intermittent fasting has several significant drawbacks for sports performance, especially for athletes aiming to optimise physical capacity and recovery:
Effects on Intense Training
One main problem with intermittent fasting is reduced performance in intense training. During fasting, glycogen levels are compromised, limiting the energy available for intense efforts. As a result, athletes may experience early fatigue and reduced performance.
Inadequate Recovery
Recovery is another critical aspect. After training, the body needs an adequate supply of nutrients, especially protein and carbohydrates, to repair muscle damage and restore energy stores. Without these nutrients, recovery is delayed, increasing injury risk.
Physiological Stress:
Fasting can increase cortisol levels, the main stress hormone, because the body perceives reduced food intake as a threat. This is even more problematic for athletes, especially at a high level, as intense exercise itself can also increase cortisol levels and temporarily suppress the immune system.
This combination of fasting, exercise and elevated cortisol can create conditions conducive to muscle catabolism, chronic inflammation and greater vulnerability to illness and infections.
Considerations for Women
It is essential to consider that, for women, the risks of intermittent fasting may outweigh the benefits. One example is hypothalamic amenorrhoea.
Calorie restriction can disrupt hormonal balance, negatively affecting the menstrual cycle and consequently the ability to recover and adapt to training. Women who play sport should therefore carefully evaluate their fasting approach and seek professional advice to avoid effects on their health and performance.
Does Intermittent Fasting Improve Athletes’ Performance?
To sum up, intermittent fasting generally does not improve sports performance, particularly in high-intensity activities or athletes aiming to maximise performance. However, for some people who have limited time to eat before exercising and are accustomed to training fasted, this practice may be viable and effective for low-intensity activities.
In these circumstances, some people may notice improved use of fat as an energy source, which may help facilitate weight loss or improve metabolic health in people who need it. However, meals eaten after these sessions must provide adequate nutrients, mainly protein and carbohydrates, to support muscle recovery and adaptation to training.
It is also essential to remember that this approach is only appropriate for light exercise. During more demanding or prolonged training, the body needs available energy stores. In these cases, the recommendation is to eat something before training, even just a few dates or a banana. Otherwise, intermittent fasting could be harmful, negatively affecting both performance and recovery.
References:
- Mattson MP, Longo VD, Harvie M. Intermittent fasting and human metabolic health. New England Journal of Medicine. 2017;376(26):2546-2560. Available at: [Intermittent fasting and human metabolic health] – PubMed
- Mattson MP, Moehl K, Ghena N, Schmaedick M, Cheng A. The effects of intermittent fasting on health, aging, and disease. New England Journal of Medicine. 2018;378(7):839-848. Available at: Effects of Intermittent Fasting on Health, Aging, and Disease – PubMed
- Tinsley GM, La Bounty PM. Effects of intermittent fasting on exercise performance and body composition in trained athletes: A systematic review. Journal of Strength and Conditioning Research. 2015;29(7):1989-2001. Available at: Effect of High-Intensity Interval Training and Intermittent Fasting on Body Composition and Physical Performance in Active Women – PubMed
- Sim KA, Partridge SR, Sainsbury A. The effects of intermittent fasting on reproductive hormone levels in physically active women. Journal of Clinical Endocrinology & Metabolism. 2020;105(3)
. Available at: Effect of Intermittent Fasting on Reproductive Hormone Levels in Females and Males: A Review of Human Trials – PubMed - Anton SD, Lee SA, Donahoo WT, McLaren C, Manini TM, Leeuwenburgh C, et al. Cortisol responses to intermittent fasting in healthy individuals: A randomized controlled trial. Obesity. 2019;27(7):1209-1213. Available at: Recurrent hypoglycemia does not impair the cortisol response to adrenocorticotropin infusion in healthy humans – Metabolism – Clinical and Experimental
