Athletes look for effective ways to manage their body weight. One popular trend is to combine exercise and intermittent fasting. Recent research indicates there may be some benefits on metabolic health, disease prevention and gut health. Studies that have looked into this have been done in healthy, moderately active people or people with some clinical condition (e.g. diabetes, obesity, high cholesterol). There’s some evidence it promotes weight loss and a reduction of fat mass too. However, there’re no studies in elite athletes.

When you search the internet, you find various approaches: fasting one day out of seven, alternating fasting and eating or fasting every third day. People are allowed to eat for a day with unlimited access to food but abstain from eating food on the day they fast. It’s important to highlight that intermittent fasting (also in most research studies) is usually done without calorie restriction.

A special form of intermittent fasting is the time restricted eating pattern. 8/16 (8 hours whereto can consume food, 16 hours of fasting) is a popular one, but there are others that are more or less restricted with a 6/18 or 10/14 window.

What's the best solution? 6/18 or 10/14?

There are different approaches of time restricted eating. The most popular one is probably the 8/16 pattern. People who follow this dietary regimen are allowed to eat food in a eight-hour-window and abstain from food in the remaining 16 hours every day. Depending on when you start your day with a breakfast, this can mean that you have breakfast at 7.00 and have your last meal before 15.00 in the afternoon. While this might work for some people, it is a challenge for athletes. Many train twice a day, some train in the evening. This means: when your workout finishes after the 8-hr-window, you spend many hours in deficit until your next calorie intake. And not just that, your recovery takes significantly longer compared to others who eat after training. With a more restricted pattern (4/20; “warrior diet”) the challenge is even bigger. Matching needs and intake is impossible for athletes who train several hours a day. It is also impossible to support training with ingestion in and around training sessions. Moreover, with such a small window there’s also another challenge, that of making it fit into a normal lifestyle. What happens when you meet with friends and can’t eat because your 4-hr-window has ended? This approach is definitely not realistic and not social.

A more relaxed approach with a 10 or 12-hr-window, however, seems to be possible. While an 8/16 approach is manageable for some, the 10/14 or 12/12 approach allows for more flexibility in the timing of food intake.

Time Restricted Eating for Athletes?

When we think of elite athletes though, they go through various training cycles. Training is intense and a good recovery requires certain steps after exercise, including the ingestion of food. An athlete’s immune system is under stress, there’s increased inflammation due to training and nutrition helps manage these metabolic situations and support overall health. If the intake is severely time restricted, it will most likely impair performance as well as health. Also, time restricted eating with a small food window in a time of intensified training will certainly increase the risk of underfueling. And remember, underfeeding (low energy availability) is an issue in sports. Athletes have a high energy demand and it is difficult to meet these needs with restricted eating.

Of course, there’s the training day when you go out fasted and do a session on an empty stomach to trigger some metabolic adaptations which are well proven in endurance athletes. But don’t forget that – when you look at all the evidence – there are a lot of benefits from the ingestion of food in and around training. More time to eat, furthermore, has another important benefit: the reduction of the risk of low energy availability.

Of note is that studies in highly trained or elite athletes are missing and so is any research project that has shown any beneficial effect of intermittent fasting on performance. So, considering the possible impairments on performance, it is too early to recommend time restricted eating to athletes with a long, intense training day. An approach like 12/12 may be possible in some situations, but there needs to be enough time to support training and recovery processes through the ingestion of food. Before applying a restricted dietary pattern, it’s important to discuss it with an expert such as a sport dietitian to make sure energy and nutrient needs are met, and performance isn’t compromised.

Additional readings:

Di Francesco et al. (2018) A time to fast. Science, 362, 770-775.

Levy & Cho (2019) Intermittent fasting and its effects on athletic performance. Current Sports Medicine Reports, 18, 266.269. doi: 10.1249/JSR.0000000000000614

Manoogian & Panda (2017) Circadian rhythms, time-restricted feeding, and healthy aging. Ageing Research Reviews, 39, 59-67. doi.org/10.1016/j.arr.2016.12.006

Moro et al. (2016) Effects of eight weeks of time-restricted feeding (16/8) on basal metabolism, maximal strength, body composition, inflammation, and cardiovascular risk factors in resistance-trained males. J Transl Med, 14:290. doi: 10.1186/s12967-016-1044-0

Peos et al. (2019) Intermittend Dieting: Theoretical Considerations for the Athlete. sports, 7, 22doi:10.3390/sports7010022