Is Intermittent Fasting Beneficial for Athletes?

woman eating breakfast

Intermittent fasting has been a hot topic for athletes of all levels in recent years. This approach to dieting is defined as eating within a certain window and fasting the rest of the time. For example, someone who fasts intermittently may eat only between the hours of noon and 8 p.m. Proponents of intermittent fasting tout its benefits—from weight loss to blood sugar control to increased mental focus—but if you're an athlete, is it worth a try?

Who Benefits From Intermittent Fasting?

Before you skip breakfast or eliminate dinner, define your athletic and health goals. Do you want a strategy to avoid late night snacking? Are you trying to be more mindful of what you're eating? Do you need to lose weight? If so, intermittent fasting might be a good fit. Some studies suggest that restricting eating to a limited time each day can have some serious health benefits. A 2018 study in the Journal of Cell Metabolism suggests that intermittent fasting can lower blood pressure, oxidative stress and increase insulin sensitivity (an important factor in diabetes prevention).

If on the other hand you're training for an endurance race like a triathlon or marathon or attempting to set a PR, intermittent fasting might hinder your fitness goals by reducing energy levels or causing you to "hit the wall" (i.e. run out of muscle glycogen stores).

Intermittent Fasting 101

If you want to give fasting a go, there are a few popular approaches. The first is the 16/8 method, where you fast for 16 hours and eat within an 8-hour window. You could achieve this by not eating after dinner and skipping breakfast. All of your calories would be consumed between the hours of noon and 8 p.m. (or whatever eight-hour stretch you choose).

Alternatively, some fasters prefer the 5:2 method in which you eat normally five days a week and fast for two days. On the fasting days, aim to consume about 500 calories total.

And finally, some people find that fasting for 24 hours a few times a month is what works best for them. Whatever approach you choose, be sure to check in with a health professional to get the all clear before starting. 

Fasted Exercise

In addition to intermittent fasting, you may have heard about the concept of "fasted exercise." The idea behind this is that when carbohydrate stores are depleted, the body relies on fat stores for energy. In turn, this may help you become more efficient at burning fat for fuel, which is key in endurance races (we're looking at you, final miles of a marathon!).

If you attempt exercise in a fasted state, choose easy aerobic workouts (e.g. an easy run before breakfast, especially if you also worked out the evening before). Keep in mind that races, long runs and speed workouts in a fasted state may result in sub-par performance because you won't have the proper muscle glycogen stores.

Fasted exercise is still somewhat controversial—some runners and coaches swear by it, but the research is still mixed. A joint position paper from the Academy of Nutrition and Dietetics, Dietitians of Canada and the American College of Sports Medicine states that "training with limited carbohydrate availability may lead to some metabolic adaptations during training, but did not lead to performance improvements. Based on the evidence examined, whereas there is insufficient evidence supporting a clear performance effect, training with limited carbohydrate availability impaired training intensity and duration."

If you decide to train in a fasted state (through intermittent fasting or other methods), make sure to refuel after exercise as your muscles are particularly receptive to carbohydrate uptake in the hour or so afterward. Keep in mind that no matter when you eat, food quality and number of calories are still important.

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