The question of whether you should eat before a run or head out on an empty stomach is almost as divisive as whether high reps or low reps are better.

If you’re trying to lose weight, working out on an empty stomach seems to make a simple, arithmetic sense: With no calories in the tank, your body should pull from its fat reserves, right?

For example: Research at the University of Bath in the U.K. found that when overweight guys walked for 60 minutes on an empty stomach, their bodies turned on certain genes that increased their rate of burning stored fat (rather than carbs), compared to guys who worked out after a carb-heavy breakfast.

Overweight and obese people respond metabolically differently to fasted exercise—usually more quickly—than fit folks, says Darryn S. Willoughby, Ph.D., C.S.C.S., director of the Exercise and Biochemical Nutrition Laboratory at Baylor University in Texas.

Active guys, too, can burn nearly 20% more fat when running on empty compared to on a healthy breakfast, according to a small study back in 2013 published in the British Journal of Nutrition.

But it’s not always that simple.

The science of fasted exercise

Exercising when you’re hungry doesn’t necessarily translate to weight loss, Willoughby warns.

In fact, as far as we know, the science says you won’t lose more fat or weight sweating on an empty stomach than a full one. There’s only been one study on this specifically, he points out: Published in Journal of the International Society of Sports Nutrition, the study found that women lost roughly the same amount of weight and fat with an hour of steady-state cardio whether they ate beforehand or not. (Gender is irrelevant, Willoughby adds—the same would apply to men.)

But fat oxidation is still good. And science shows a few other perks to running on empty, too: Healthy, untrained men and women significantly increased their VO2max (a measure of cardiovascular capability) when they trained in an overnight-fasted state compared to eating a small amount of carbs beforehand, according to a tiny study in the Journal of Science and Medicine in Sports. Meanwhile, cyclists improved their power without compromising endurance performance when they did training rides after an overnight fast, says research in the Journal of Strength and Conditioning Research.

When fasting exercise works—and when it doesn’t

After fasting for eight to 12 hours—overnight, for example—and heading out the door, your body first wants to tap into the carbs (the glucose, really) stored in your muscles and liver for energy, Willoughby explains. Once your carb stores start running low, your body turns to burning stored fat, but this takes longer to convert into energy.

Here’s where that gets complicated.

If you’re sweating at a light to moderate intensity for less than 90 minutes, your body should have a fairly easy time transforming fat into energy, and you can have a great workout without fueling up beforehand. “Most people have enough glycogen to fuel them for that duration of exercise, and fat is also being mobilized and oxidized at a faster rate than carbohydrates at this lower intensity,” explains Katie Kissane, R.D., C.S.S.D., a sports nutritionist and the owner of My Nutrition Coach in Fort Collins, CO.

But if you’re doing a high-intensity workout, your performance will suffer from skipping carbs beforehand. (If you’re religiously on a high-fat keto-style diet, you may fare slightly better, but the research on this is still relatively thin.)

“It is not possible to mobilize fat stores and burn enough fat during higher-intensity exercise to provide a steady state of fuel, so once the body runs out of carbohydrates, it will no longer be able to continue working at that intensity,” says Kissane.

This was borne out in a small study in the British Journal of Sports Medicine. In the study, moderately trained guys ran for 60 minutes; during the last half-hour, the runners ran much more intensely. The study found that fasted runners weren’t able to run as far, or as fast, as guys who had eaten.

Furthermore, your body doesn’t just burn fat during workouts—it can also burn fat afterward, a phenomenon known as excess post-exercise oxygen consumption. Eating beforehand might reduce your mid-workout fat burn, but bonking halfway through high-intensity training means you’re giving up that post-sweat fat burn that comes when a grueling workout sends you into EPOC. Plus, if you end a workout feeling depleted, you increase your risk of overeating later in the day, Kissane adds.

Another strike against working out on an empty stomach: Fasting before workouts puts more stress on your body—and that stress causes the release of cortisol, which leads your body to break down muscle to use protein as fuel. “Doing fasted cardio can impact fat oxidation, but, unfortunately, it also typically impacts protein degradation, which is not necessarily a good thing,” Willoughby explains.

The final word

If you just want to lose fat, your priority is fat oxidation, so you want to go into steady-state cardio fasted. But if you want to lose fat and maintain muscle, you should fuel up before.

There is one trick to burn fat while eating beforehand: Eat protein (like a whey shake) 30 minutes before you start sweating, Willoughby advises. The protein will provide some fuel to your body, but protein doesn’t spike your insulin levels as much as carbs will—which means your body will have fuel, but without the blunted fat burn.

As far as keeping your energy up, if you head out on a run or a ride first thing in the morning, you’re fine to go in an overnight fasted state as long as you keep it under 90 minutes. If you’re hitting the cardio later in the day, Kissane recommends eating a small snack two hours before any workout.

If you’re about to hit it really hard—either moderate-intensity cardio longer than 90 minutes or a high-intensity workout longer than 30 minutes—fuel up about 30 minutes out, both experts agree. A protein shake will keep your body from breaking down muscle in order to use fuel for energy, but you may need the quick energy that comes from carbs, even if you also lose that fat oxidation.

A better option: Mix carbs and protein, like a slice of toast with almond butter or half a serving of oatmeal made with milk, Kissane recommends.