You Asked: Is It Bad to Exercise on an Empty Stomach?

Are you the type to squeeze in a morning run before breakfast? Or hit the gym before dinner? Working out on an empty stomach won’t hurt you—and it may actually help, depending on your goal.

But first, the downsides. Exercising before eating comes with the risk of “bonking”—the actual sports term for feeling lethargic or light-headed due to low blood sugar. “You might feel tired or edgy, and you won’t be able to work out as intensely as you would have if you had eaten something,” says Douglas Paddon-Jones, a muscle physiology researcher and a professor of aging and health at the University of Texas Medical Branch. “A light amount of food will help you get through the workout session more comfortably and with more energy.” (Here’s what to eat before a workout.)

It’s extra important for older adults over 55 to eat something before exercising—especially when working out first thing in the morning. “Throughout the night, our body makes physiological adjustments so we’re able to survive, and that includes breaking itself down to sustain our blood-glucose levels,” says Nancy Rodriguez, a professor of nutrition sciences at the University of Connecticut. If you exercise in the a.m. before eating, she explains, your body continues to operate in this “break-down state,” which can lead to muscle loss—a concern for many seniors.

But if your primary exercise goal is weight loss, exercising on an empty stomach may come with some promising perks—though more research is still needed.

“There’s some research to support the idea that working out in a fasted state can burn more fat [than exercising in a fed state],” Paddon-Jones says. One 2013 study of 64 people with obesity found that fasting every other day—eating just 25% of their typical daily calories—while doing an aerobic training program led to greater weight loss than diet or exercise alone.

But the evidence is mixed. Some studies have also failed to find weight-loss advantages associated with fasting when compared to traditional diets. Other research suggests that skipping breakfast may be bad for your health. Even when fasting studies found benefits, the experiments were tightly controlled, and the people in them had help from nutrition scientists to ensure they got all the nutrients their bodies needed, Paddon-Jones points out.

The hype about fasted cardio may also not prove true for everyone. “You see some people advocating for fasted exercise on blogs and lifestyle sites, but these people tend to be young and in amazing physical shape,” he says. “Every other part of their diet and lifestyle is dialed in, they have no health issues, and when they eat, what they’re eating is super-high quality and carefully regulated.” On the other hand, when an “average Joe” attempts a fasted-cardio plan without proper planning or professional oversight, he may run the risk of nutritional deficiencies or unhealthy body composition changes, Paddon-Jones explains.

Again, adults over 50 need to be careful fasting before a workout. “Protein recommendations tend to be higher for older adults, and that protein may be more beneficial when spread throughout the day,” says Shivani Sahni, director of the nutrition program at Harvard University’s Institute for Aging Research. “I think a lot more work needs to be done on intermittent fasting and exercise before we can say, ‘yes, this works well for this type of individual.’”

For people who aren’t athletes, fueling up with food before a workout may be the best way to get the most fat-burning benefit. Rodriguez recommends a small pre-exercise snack that combines protein and carbohydrates. “Have a half a banana with a spoonful of peanut butter, or a hard-boiled egg,” she suggests. You don’t want to fill up; you just want to give your body a burst of energy to power your training session.

After you exercise, drink some water—but wait 60 to 90 minutes before eating. “You’re trying to take advantage of your elevated metabolic rate post-exercise, so if you can go an hour or an hour and a half without eating, you’ll maximize the fat-burning response,” she says.

This is general advice, of course. If you’re body-building or training for a marathon—or if you have a metabolic disease like diabetes—you should speak with a doctor or registered dietitian to design a plan that suits your needs.