Is it ok to exercise while fasting?

Absolutely. In fact, combining fasting with endurance exercise can produce superior changes in body weight, body composition and lipid indicators of heart disease risk, compared to either exercise or fasting alone.

Contrary to popular belief, it is safe to exercise on alternate fast days or during fasts not exceeding 24 hours. In fact, in studies of IF, research participants often report boosts of energy on days when they fast or eat fewer than 500 calories.

“In the first 10 days of an alternate day fasting practice, you may notice decreases in energy or concentration levels,” Dr. Krista Varady says. “But after these first 10 days, most people find it easy to still exercise on their fast days.”

In one of the first studies to combine fasting and exercise interventions, Varady ran a 12-week study of intermittent fasting combined with endurance exercise (brisk walking / cycling). Obese research participants on an alternate day fasting schedule came into the lab three days per week to exercise on stationary bikes and treadmills. When given the option of training during either “feast” or fast days, participants chose to exercise on fast days just as often as on feast days.

If your primary goal is weight loss, however, you may also want to wait until after exercise to eat during an alternate fast day, according to Varady’s study. Participants who consumed their one allotted fast day meal before exercise tended to get a burst of hunger an hour or so after exercising, leading them to often cheat and eat extra calories that day.