Illustration by Sam Woolley.

If you’ve ever tried to hit the gym after a heavy meal, you probably know that it doesn’t feel great and you probably wish you’d just skipped the pre-workout snack. But that doesn’t mean you should workout on a totally empty stomach. Here’s why you’re probably better off fueling up before you go for a run or crank out some super sets at the gym.


The science

Let’s say it’s time for your daily workout. Do you have some food first? Or do you put it off until after you’ve finished your usual regimen? Proponents of exercising while “fasted,” or in a state of hunger, suggest you can speed up your weight loss by doing so. One study published in the British Journal of Nutrition found that participants who exercised while fasted burned nearly 20 percent more fat than those who ate before beforehand. And another study, published in the International Journal of Sports Nutrition and Exercise Metabolism, had similar findings, suggesting aerobic training in a fasted state lowers more body fat percentage in addition to body weight when compared to fed training.


Why the extra fat burn? As sports dietetics specialist Kelly Pritchett, Ph.D., R.D., puts it, your body is switching fuel sources. To perform intense physical actions like long-distance running or lifting weights, your body has to burn glycogen, or your stored up carbohydrates. If your body runs out of glycogen reserves—like it would in a hungry, fasted state—it has to use something else to keep you going. In this case, all that excess fat you’re hoping to get rid of.

But there’s a catch—well, a few of them. For starters, your body isn’t a fan of being starved, and it likes having fat stores. When you burn fat rapidly, your body begins to adjust your metabolism to compensate for that loss. Basically, it goes into a kind of survival mode and starts to burn fewer calories, says Pritchett. By burning so much fat, your body thinks it needs to store more of it when eat your next meal, completely counteracting those fat burning benefits. And as another study in the American Journal of Clinical Nutrition found, extended periods of fasting can lead to a drop in resting metabolic rates. So there’s no real upsides to foregoing food in this type of scenario.


Pre-fueled exercise may also suppress your appetite more than fasted exercise, according to one study published in the journal Appetite. All in all, participants expended the same amount of energy per day, but those who ate before working out felt less of an urge to eat more later. Considering the fact that weight loss largely comes down to how you eat, not what you do, this can be an important factor for those exercising to lose weight.

What it does to your muscles

Furthermore, fasted exercise can cause you to shed some muscle in addition to fat under some circumstances. If your body has burned through its glycogen fuel stores, it may also obtain energy by breaking down muscle proteins in addition to those fats. Intense training always breaks down muscle so it can get stronger through protein synthesis, but doing so while fasted harvests more muscle sooner, making it more difficult to recuperate the lost mass.


That said, this only occurs when you increase the intensity of your workout beyond your normal routine. If, say, you were to do a normal workout first thing in the morning, your muscles would probably still have sufficient glycogen stores leftover from your previous meal. And according to one study from the Journal of Applied Physiology, being in a fasted state doesn’t increase or decrease your physical output or exertion during typical, submaximal exercise. Essentially, whether you workout hungry or not, you can do your usual thing at the usual intensity without worrying about muscle loss.


Not your best workout

That can lead to another problem, though. While you could get through a workout fasted, you won’t have the energy to push yourself harder, and thus, it’s unlikely you’ll be able to improve at the same rate were you to eat first. A separate study, published in the Journal of Science and Medicine In Sport, suggests that fasted training significantly underperforms fed training when it comes to maximal exercise, or reaching for new personal bests. Pushing yourself is what leads to increased muscle mass and faster run times, so you want to do it as much as you can. When it’s all said and done, eating before a workout can actually increase your metabolism in the long run.


In short, yes, it’s feasible to work out on an empty stomach and get by just fine. Some people prefer it because they feel lighter, are more alert, and experience increased focus. But beyond personal preference, there’s not many benefits. You’re better off eating a carb-dense meal about an hour before your workout, and following it up with a high-protein, light-carb meal afterward. Plus, not eating puts in you in a crappy mood. Nobody wants to deal with a jerk at the gym. And you’re better off finishing your workouts feeling happy and refreshed, not miserable and ready to kill for food.


This story was originally published in 2017 and updated with new information on 2/20/2020.