Fasting and Exercise: The Perfect Match for Better Health

Diets that include some form of fasting are becoming increasingly popular. Now, new research suggests that food deprivation might actually boost physical endurance.

Athletes who train for endurance events often consume large amounts of food, particularly carbohydrates, to produce the glucose that fuels their muscles. Others prefer a more controlled diet. But what about fasting? Bodybuilders, cyclists, and other athletes who occasionally abstain from eating are on the rise. Some fast two days a week, consuming around 600 calories (not a true fast, but enough to achieve similar metabolic effects), while eating normally the other five days. This is the so-called 5:2 diet. Meanwhile, they engage in a full workout regimen that includes both aerobic and strength training.

At first, this might seem counterintuitive, as exercise consumes energy that needs to be replenished through food. So how can fasting work? Recent research conducted on mice, published in late February on the Federation of American Societies for Experimental Biology’s website, suggests that fasting and exercise can actually go hand in hand and even increase endurance.

According to Mark Mattson, from Johns Hopkins University and Director of the Neuroscience Laboratory at the National Institute on Aging in Baltimore, it makes perfect evolutionary sense that fasting and physical activity would have a synergistic effect. It’s very likely that our ancestors went long periods without eating and hunted on an empty stomach, he argues. “When fasting, those with a well-functioning brain and body had better chances of survival,” says Mattson, who is also a co-author of the study on mice.

In the experiment, the mice were divided into three groups: the first group ate as much as they wanted and ran for 45 minutes a day on special treadmills, the second group fasted every other day and also ran for 45 minutes, and the third group fasted every other day but did no exercise. A control group ate as much as they wanted without ever running. The diet was high in carbohydrates, and the test lasted two months.

The mice that fasted and ran lasted longer on the treadmill than all the others, in some cases up to 30 percent longer, despite consuming 10 to 15 percent fewer calories than the sedentary mice. “The most important result is that intermittent fasting in conjunction with daily exercise increased endurance,” says Mattson.

The study also cites a group of professional cyclists who performed better when they fasted in the evening (and skipped breakfast) compared to when they didn’t fast. Additionally, the study’s authors write, for those who fast regularly or follow a diet, strength exercises can prevent muscle loss by increasing the oxidation of fatty acids in the cells. During a 12-16 hour fast, the body consumes the glucose stored in the liver and begins to use fatty acids for energy. Instead of burning sugars, it burns fats—a more efficient energy source—by activating ketone bodies.

Metabolic Shift

Other studies on animals and humans suggest that the transition from glucose metabolism to lipid metabolism is associated with better health and increased resistance to chronic diseases. And according to some cognitive studies, when ketones are activated, the brain functions better.

In recent years, ketones have entered popular culture to the point that many celebrities follow the ketogenic diet, which is low in carbohydrates and high in fats. Some believe that this diet mimics the benefits of intermittent fasting by forcing the body to use fats as an energy source. However, not everyone agrees that the ketogenic diet increases endurance, and many dietitians are skeptical of its nutritional value.

Michelle Harvie, a nutrition researcher in Manchester, UK, helped design the 5:2 diet but has doubts about the ketogenic diet: it does lead to weight loss, but it lacks fiber and is rich in saturated fats, which can cause cardiovascular diseases. “And there’s growing evidence of its negative effects on the gut microbiome,” she explains.

As for intermittent fasting, Harvie hopes for a revolution: “In the UK and the US, fasting doesn’t exist. People eat continuously, even at night. That’s why I think it’s necessary to return to some form of fasting.”