Intermittent fasting is the practice of restricting calories on one or more days of the week or going without eating for slightly longer than normal periods of time within the day. It has gained some popularity among dieters and bodybuilders seeking to shed body fat, and a growing number of endurance athletes (especially ultrarunners) do it to increase the fat-burning capacity of their muscles, which, in theory, increases endurance.
Endurance athletes often ask me for my take on intermittent fasting. Although some research suggests that it can be an effective way to lose weight, and although some swear by it, I discourage the practice. Here are my four reasons advising endurance athletes to give intermittent fasting a pass.
1. There are simpler ways to lose weight.
In a recent review of past research comparing the effects of intermittent fasting and continuous calorie restriction, Australian scientists concluded, “Intermittent fasting…represents a valid—albeit apparently not superior—option to continuous energy restriction for weight loss.”
In other words, intermittent fasting works as well as, but no better than, the traditional weight-loss method of eating a little less each and every day. But if intermittent fasting doesn’t work better than continuous calorie restriction, it is more complicated. One version of intermittent fasting entails choosing two days of the week on which to eat lightly (500 to 600 calories) and eating normally the other days. Another version entails choosing two days of the week on which to eat normally and fasting for 16 out of 24 hours on the other five days.
That’s a lot of numbers and planning and switching back and forth. Why not just consistently avoid overeating every day if the results are likely to be the same?
2. Fat burning is overrated.
Scientists have not rigorously investigated whether intermittent fasting significantly increases the ability of the muscles to use fat as fuel during exercise. But even if it does, this physiological effect is unlikely to translate to better running performance. Research involving low-carb diets—a more popular way of increasing fat-burning capacity that is proven to work—has shown that this method fails to increase performance in multi-hour endurance events and actually impairs performance in events lasting less than about 90 minutes. So there’s no reason to expect that intermittent fasting would do any better.
The most relevant study conducted to date was one that looked at the effects of Ramadan fasting on time trial performance in middle-distance runners. During the month of Ramadan, Muslims fast between sunrise and sundown, a schedule that is similar to that of many runners who practice intermittent fasting. On average, the subjects’ 5000-meter performance dropped by 5 percent between the beginning and the end of that month intermittent fasting.
3. It encourages a magic-bullet mentality.
In my work as a sports nutritionist, I have found that some athletes have a magic-bullet mentality. They are always looking for some revolutionary new dietary method to give them the results that they have so far failed to achieve. But I believe it is this very mentality that prevents such athletes from getting the results they seek.
There is simply no need for revolutionary new dietary methods. The nutritional practices that really work—eating a balanced, varied, and inclusive diet, for example—have existed forever. These tried-and-true practices are how the most of the world’s greatest runners—the gold medalists and record breakers—get their results.
Think about it: If intermittent fasting was truly necessary to achieve an optimal body composition and maximum fitness, there would have been no endurance athletes with an optimal body composition and maximum fitness until intermittent fasting was invented!
4. It’s not normal.
I believe strongly in keeping the diet as normal as possible in the pursuit of health and fitness goals. Put another way, I believe in changing one’s existing eating habits as little as necessary to achieve one’s health and fitness goals.
There are a few reasons for this. First, an altered diet is easier to sustain if it retains some familiar and preferred habits and patterns. Second, an altered diet is less disruptive to a person’s social relations if it remains more or less culturally normal. And third, extreme and radical diets of all kinds often serve as stepping stones toward disordered eating for the susceptible.
Now, make no mistake: If your current diet is very bad, you will need to make significant changes in order to achieve your health and fitness goals. If, for example, the only vegetables in your current diet are french fries and ketchup, you are going to have to make room for at least two or three servings of real vegetables every day.
But there is never any need for more severe measures such as eliminating meat and fish from the diet, limiting carbohydrate intake to 100 grams a day or less, or practicing intermittent fasting. Such measures do sometimes work out for some runners, but the odds of success are much greater with the more conventional eating practices favoured by most of the best athletes in the world.