If you follow fitness accounts on social media, chances are, you’ve heard of fasted workouts, which, true to their name, involve exercising without eating beforehand. Those who swear by the practice typically knock it out in the morning — after fasting while asleep — and tout it as a way to burn more fat and lose more weight. Of course, people exercise for a multitude of reasons, which may or may not include shedding pounds. But if weight loss is a goal of your fitness routine, you might be wondering whether fasted workouts can work.
As with so many wellness trends, there’s a lot of hype surrounding fasted workouts. The reality is, research has shown that diet, not exercise, is what’ll really help you burn fat and lose weight, Kerry Stewart, director of clinical/research exercise physiology and professor of medicine at Johns Hopkins Medicine, tells Mic. Your workouts can help you maintain the weight you’ve lost from adjusting your diet — but they needn’t be fasted to do so.
Before we dive too deep into the science, let’s go over what happens in your body when you work out. During exercise, you burn mostly stored fat and blood glucose, or blood sugar. If you’re new to working out, then your body will burn mostly glucose, leaving you dizzy and struggling through your workout, or forcing you to stop altogether. As your fitness improves, though, your body makes better use of the fat it’s stored up, burning more fat and less glucose to do the same workout, preserving glucose so that now you can exercise harder, longer, or both.
But even if you’re pretty fit, and therefore burning more fat during exercise, “it’s been very, very clear through lots of good research that exercise is not the primary way to lose weight,” Stewart explains. Diet is. A low-carb diet can be effective for weight loss, since it denies your body of carbohydrates so that it burns stored fat to fuel your daily activities instead. So can lowering your caloric intake, which similarly forces your body to burn stored fat as fuel to make up for the caloric deficit you’ve created.
It makes sense that diet matters more for weight loss than working out. “You’re not burning a lot of calories from exercise,” Stewart says. To put this in perspective, you need to walk three or four miles to burn off the 300 to 400 calories in a bagel. You actually burn more calories throughout the day from routine activities like walking, cooking, and sitting at your computer.
That said, exercise can help you maintain the weight loss that results from changing your diet, so that “if you’re exercising as you go along with the diet… you’ll probably be able to keep the fat from accumulating again,” Stewart says. But given that exercise plays more of a supporting role in fat and weight loss, it doesn’t really matter whether or not you do it on an empty stomach.
If you want to give fasted workouts a shot, keep them relatively short, Stewart says. For any activity lasting longer than hour, you’ll want to eat something in order to avoid hypoglycemia, or low blood sugar, which can cause you to faint. (Think of it as not having enough gas in the tank before you drive on the freeway.) Also, listen to your body. If you ate a filling dinner the night before and don’t usually eat breakfast, a fasted workout lasting less than an hour will probably be fine — but stop as soon you feel weak or lightheaded, which could indicate hypoglycemia.
Basically, fasted workouts won’t make or break your weight loss goals, so don’t force yourself to do them if you don’t think they’re for you. “I don’t think the general public, someone not training for an [athletic] event, needs to think about these little fads that people come up with, be it supplements, this vitamin, that vitamin, or fasted workouts,” Stewart says.