Is Intermittent Fasting a Good Idea for Athletes?

Breaking your record time may have less to do with how you train and more to do with how you eat. Fans of intermittent fasting praise the eating style for its weight control benefits but according to a new study, it may also improve your performance.

For a six-week period, researchers had 12 male athletes participate in an every-other-day fasting program, similar to what is known as the 5:2 method. On fasting days (a total of three days per week), they were restricted to 600 calories. On non-fasting days, they resumed their normal eating patterns, which came in at 2,351 calories per day. Before the implantation of the calorie reduction, each participant was given an incremental maximal test until exhaustion on a treadmill to measure their maximal oxygen consumption (VO2max) at the beginning of the study and at the end.

Researchers discovered that after the six-weeks of calorie restricting, participants’ heart rates were lower when performing the same activities at the same level of power than at the start of the study. Their bodies became more energy efficient. They reported less fatigued. They also had lower blood lactate levels and showed an improved tolerance for workloads, meaning their body could handle more stress. The men also lost a significant amount of body fat (15.1 percent) while only losing a minimum of muscle mass (2.91 percent).

If you’re not familiar with intermittent fasting, here’s a brief overview:

As the name implies (the word intermittent means ‘coming and going at intervals; not continuous’) it’s a form of eating where one only consumes food during specific windows of time. There are several different versions, the most common being the 16:8 method (eat for 8 hours, fast for 16), the Eat-Stop-Eat method (doing a 24 hour fast, up to 2x per week) and the 5:2 method (reduce calories to 500-600 two days per week).

While fasting is not a new concept, the idea of it as a lifestyle is. Like most diets that hit the mainstream, it’s been touted as a cure-all for a variety of common concerns. Along with weight management, intermittent fasting has been said to improve insulin sensitivity, memory function, and even reduce the negative signs of aging. Unfortunately, when you look at the scientific evidence available, you find that many of the cited studies are either small, have been performed on rodents or require further investigation before definitive conclusions can be made. Some of what’s been found, such as the possibility of reducing chronic disease risk, as reported in Cell Metabolism, is promising. However, this doesn’t necessarily mean you should jump on the bandwagon.

Claire Shorenstein, a NYC-based Registered Dietitian Nutritionist and Road Runners Club of America certified running coach, advocates a more traditional nutritional plan for her clients. On her website Eat for Endurance, she encourages choosing whole foods and creating balanced meals that satisfy, fuel workouts and aid muscle recovery. When it comes to intermittent fasting, she warns runners to me mindful about their energy intake.

“Under-fueling, and as a result increased risk of injury and illness, is a major concern for athletes following an intermittent fasting plan,” she explains. “With the 5:2 method, you may not be getting in enough nutrients given that you’re barely eating two days out of the week, not to mention that exercising on those days will likely result in poor performance due to low energy levels and inadequate recovery nutrition. Yes, you can take a supplement, but you can’t capture all the beneficial nutrients found in food within a pill.”

With other forms of intermittent fasting, there are similar concerns. “With the 16:8 method, many people end up skipping breakfast, which is not a good idea if you exercise in the morning,” Shorenstein explains. “Training on empty is fine if you’re doing a relatively short (<60-90min) and/or low to moderate intensity session, but not smart if you’re exercising for a longer period of time and/or at a higher intensity.”

This isn’t to say that intermittent fasting should be disregarded, only that adequate research and assessment of your own needs and goals should be done before diving in.  While Shorenstein doesn’t recommend it for training for an event, she points out that it may be more sustainable during less active periods. “There is some interesting research emerging on intermittent fasting generally speaking, but it still remains spotty,” she says.