Fasting meets exercise

How the eating strategy can complement your fitness goals

For the past few years, a growing number of people have been strategically rejiggering mealtimes, and the approach is only becoming more mainstream. Scientists are still uncovering exactly how it affects your body, but intermittent fasting—when you don’t eat for periods of time longer than a typical overnight fast, alternated with bouts of normal eating—may help you shed fat while holding on to your lean body mass, says Dayton, Ohio-based sports nutritionist Pamela M. Nisevich Bede. Though it’s not the right strategy if you’re trying to gain muscle, notes Grant Tinsley, Ph.D., an assistant professor of exercise physiology at Texas Tech University in Lubbock, Texas.

You can maximize the benefits by combining intermittent fasting with strength training two to four days a week and getting enough protein (1.4 to 2 grams per kilogram of bodyweight per day), Tinsley says. In his study, participants who paired time-restricted fasting with a resistance training regimen ate less overall and saw no harm to their muscular performance.

Make fasting work for you by choosing a style that fits with your schedule. Here’s a sampling of the most popular methods—and how to plan your workouts accordingly.

Time-restricted fasting
On these plans, you extend your overnight fast, confining your eating window. For example, you might fast for 16 hours (say, from 6 p.m. to 10 a.m.) and then eat within eight hours (from 10 a.m. to 6 p.m.). Ideally, you’d work out during your eating hours, says Tinsley. This way, you have fuel on board to power your performance and you maximize the metabolic effect of exercise when you eat right afterwards, says Mercer Island, Washington-based registered dietician Susan M. Kleiner, Ph.D., author of The New Power Eating.

If you can’t make that happen, you’re better off exercising at the beginning of your fast, rather than at the end when you’re hungry, advises Tinsley. When you must work out towards the end of your fast (like first thing in the morning), stick to a lower-intensity session. Have some coffee before you start and eat as soon as possible afterwards.

To make things less complicated, you can move your eating window around from day to day to fit your needs, and you don’t need to do time-restricted fasting every day for it to work, says Tinsley. So you can schedule higher-intensity workouts for non-fasting days. “It’s very individual how many days you need to do it to see results,” he says. “I encourage people to start out conservatively and then experiment.”

Alternate day fasting
Another popular strategy is to switch off between days of very restricted eating (for example, 500 calories in 24 hours) with days where you eat normally, explains Bede. You’ll want to save tough workouts for non-fasting days, but you can still do light cardio or mobility work on those low-food days, she says. “Trust your body, and see what you feel up to.”

Also, it doesn’t matter when you eat those 500 calories within the day since the goal of any movement is not muscle-building. “This can be a successful strategy for people who like fasting, but it’s not for everybody,” says Kleiner.

Full day fasts
Other people may try fasting for 24 hours just once a week on a day they’re not training, says Kleiner. This is definitely not a time to exercise, but you can use that day for active recovery, she says. Be mindful of your limitations, though: For example, while yoga may be okay for some, others may find their balance affected.

When to skip it
Part of the appeal of an intermittent fasting diet is that it takes some of the decision-making out, says Kleiner. “People like it because when they’re fasting, they don’t have to make any choices around food. And when they’re not fasting, they can be a little more relaxed.” But if you have a complicated schedule, the juggling might have the opposite effect. “Be careful that it’s not causing you distress,” she warns. And if you find yourself unable to get through your workouts—particularly if you have an important fitness goal—realize that this eating style may just not be for you, adds Bede.