Intermittent fasting — periods of voluntary abstinence from food and drink — is an ancient practice that has been followed in a variety of different ways throughout the course of history . This process has become a viral topic that has drawn attention from amateur to professional athletes due to its proposed effects on weight reduction and possible performance benefits as a result.
Though it is a popular term that has been coined to describe periods of fasting, there are many different types of intermittent fasting.
Some of these include: alternating days of fasting and eating also known as complete alternate-day fasting; modified fasting regimens which can involve severe energy restriction for two non-consecutive days per week and eating freely for the other five days; time restricted fasting which involves eating freely within specific time frames and religious fasting such as a fast from sunrise to sunset during the holy month of Ramadan .
Though these are popular dietary regimens in the general population, the bulk of scientific evidence for the health benefits of intermittent fasting comes from studies of male rodents. Human studies have been limited to observational studies of religious fasting, health outcome data, and studies with small patient populations . Although nutrition recommendations for triathletes have been widely published, there has only been limited data reported on the performance affects regarding intermittent fasting.
Hyperthermia, dehydration and carbohydrate depletion can all affect performance, which can be mitigated with adequate nutritional measures . Chowdhury et al.  did not observe differences in 24-hour physical activity in a group of athletes that skipped breakfast compared to those athletes who did not. When looking at the body composition data and lipid profile, there are some interesting results with intermittent fasting.
Severe energy restricting and alternating-days of fasting have been shown to reduce body weight, body fat, triglycerides, and cholesterol  in normal weight, overweight, and obese adults. Though there are no studies evaluating the available energy for athletes on an intermittent fasting regimen, this article will review the available studies evaluating different components of a triathlon and the potential effects intermittent fasting can have on them.
We will first focus on swimming. In the current literature, there is one study that investigated the effects of intermittent fasting and swimming on body composition and lipid metabolism in humans. The authors reported that combination of intermittent fasting and moderate aerobic exercise is capable of modifying lipid metabolism, and can promote changes in body composition inducing weight loss with maintenance of lean body mass . They did not report any performance benefits with swimming itself.
In terms of cycling data, intermittent fasting reduced anaerobic leg power over 30 seconds during the early phase of restriction (around day two), but performance returned to that seen at baseline from day four onwards . This initial reduction may suggest there is a transition between the acute and prolonged practice of intermittent fasting. No performance benefit was seen after returning to baseline. Though there is existing literature regarding anaerobic performance changes with intermittent fasting, we could find no studies that highlighted aerobic performance changes in cycling or endurance events.
Finally, even though no studies are available that evaluated the effects of fasting on endurance running performance, we did find one published by Cherif et al who evaluated the effects of intermittent fasting on sprinting performance. They found that intermittent fasting impaired speed and power in their study. Though performance was affected, it should be noted that cholesterol levels for the study participants improved similar to the body composition data that was mentioned in prior studies .
In a sport where body finishing time can be correlated with body mass index (BMI), it would seem that lowering BMI potentially safely via intermittent fasting may also result in an improved performance. Unfortunately, considering all the factors we have discussed, the effects of intermittent fasting on triathlon performance are unknown.
The studies that we mentioned involved athletes at different levels of fitness who were participating in one specific activity such as swimming, cycling, or running, unlike the population of athletes seen in a triathlon. We will have to wait and see if more data can shed more light on the performance benefits of intermittent fasting for triathletes.
However at this time, we have no recommendations for or against intermittent fasting. In terms of the body composition data, intermittent fasting may improve body weight, body fat, triglycerides, and cholesterol levels in athletes who choose to follow that dietary strategy.
Jason Diehl, MD is a sports medicine physician practicing at OhioHealth Sports Medicine in Columbus, Ohio. His practice focuses on endurance athletes, and he has been one of the USA triathlon team physicians for 9 years. Dr. Diehl was also a member of Team USA in Odense, Denmark 2018.
Niraj Patel DO is sports medicine fellow at Riverside Methodist/OhioHealth Sports Medicine in Columbus, Ohio.
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The views expressed in this article are the opinion of the author and not necessarily the practices of USA Triathlon. Before starting any new diet or exercise program, you should check with your physician and/or coach.