Intermittent fasting for Athletes

29th July 2024AnitaArticles, Blog, Sports Nutrition, Sports nutrition

Intermittent fasting continues to be hyped by celebs and influencers but is it an effective strategy for athletes and active people?

Recently, interest in intermittent fasting (alternative periods of eating and abstinence) has rocketed. The 5:2 diet, in which you eat normally for 5 days a week but limit calorie intake on two, was popularised by Michael Mosley after a 2012 documentary. Since then, other approaches have gained traction, including ‘time restricted eating’ (TRE) where you eat in a consistent window of time each day, typically 8 – 12 hours, and fast for 12 – 16 hours.

Potential benefits

The idea behind TRE is to give your digestive system ‘rest’ time – time to switch its priorities away from digestion and onto other functions, such as repair. Advocates claim it can help you lose weight, prevent chronic disease, increase longevity and improve the body’s ability to utilise fat during exercise.

While it is not specifically a weight loss diet, many people find they lose weight. A review of 27 studies published in 2020 found that people who limited their eating window lost 0.8 – 13% of their body weight over 2 – 12 months. This may be explained by the limitation imposed on eating opportunities. Many people find they feel less hungry or have fewer opportunities to eat.

However, there is little evidence to suggest intermittent fasting is more effective for weight loss than other approaches. A systematic review of 40 clinical trials showed that intermittent fasting resulted in equivalent outcomes in terms of weight loss, fat loss and insulin sensitivity (how effectively the body regulates blood sugar) compared to daily caloric restriction.

Most of the studies indicating a health benefit of intermittent fasting have been done with lab animals. There are relatively few human trials to date. Of these, studies point to improvements in insulin sensitivity and blood sugar control, which may help reduce the risk of developing type-2 diabetes, as well as reduced inflammation. It may also benefit gut health. In a study published in the British Journal of Nutrition, people following the 16:8 approach saw a significant increase in their overall microbiome diversity.

The downsides

Although intermittent fasting has been shown to reduce body fat and maintain lean mass in athletes, these changes do not always improve performance. For example, a 2020 study of elite cyclists showed that four weeks of TRE resulted in weight loss (2% of body weight) but no improvement in aerobic capacity or performance compared to those who ate normally. Similar results were shown in runners who followed a 16:8 TRE diet for four weeks: they lost weight but did not improve their running performance. In a 2018 meta-analysis of 46 studies, researchers concluded that intermittent fasting had no effect on performance.

Men and women who followed a strength training program had similar gains in muscle mass and strength when practising intermittent fasting compared to a control diet.

A big concern around intermittent fasting for athletes, particularly those undertaking high volume training is that it may result in a significant energy deficit. If the deficit is prolonged this may increase the risk of RED-S, which can have an adverse effect on an athlete’s health and performance.

Fasting and the female body

Fasting together with a strenuous exercise routine can be very stressful for our bodies. According to Dr Stacey Simms, exercise physiologist and author of Next Level, intermittent fasting is not a good idea for women who exercise regularly. “As a woman, your body will fight to preserve energy when you start fasting. When your brain perceives nutrient deficiency, especially a deficiency of carbohydrates, there is a marked reduction in kisspeptin [a neuropeptide] stimulation, which not only increases your appetite but also reduces your sensitivity to insulin. This is why research shows intermittent fasting is more likely to cause impaired glucose intolerance in women than in men.” According to Simms, if you exercise while fasting, it creates a double whammy of stress, raising cortisol levels and depressing thyroid activity, both of which disrupt the menstrual cycle.

Summary

While intermittent fasting may have weight loss and health benefits for the general population and moderately active people, I would not recommend it to highly active individuals or athletes undergoing hard training. This goes especially for female athletes and active older women. Overall, the risks of intermittent fasting outweigh the benefits. Going for long periods without eating can lead to fatigue, under-fuelling and nutrient deficiencies. On the other hand, if you are doing relatively light training and want to give it a go, ensure you eat enough to fuel your day and overall training.

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