On a Scale of 1 to Yikes, How Bad Is It if I Work Out On an Empty Stomach?

Like most personal exercise and nutrition habits, what and how much you eat before a workout is exactly that: personal. “It really depends on the individual,” says Kari Lansing, CTSC, an athletic trainer and strength and conditioning coach based in Lake Placid, New York. Depending on your everyday diet, your body type, and your fitness goals, hitting the gym hungry every once in a while is probably fine. But long-term—especially if you’re exercising for extended periods of time, like training for a marathon—it could have some serious health implications. “I always recommend having fuel in your system,” Lansing says.

Below, Lansing and other experts break down the ins-and-outs of working out on an empty stomach: why the right food is so important for exercise, the short- and long-term implications of not having enough fuel in your tank, and recommended pre-workout snacks.

How long is too long to wait between eating and exercise?

Lansing says that if your last meal was three to four hours before a low-intensity workout, that might suffice. But if you’re hitting the gym hard after, say, a night’s sleep without eating—or you're coming off of an intermittent fast? That’s not so good.

“It’s our body’s primal need to keep maximum efficiency,” says Lansing, who’s been training professionally for 24 years and recently founded the online women’s running community Bring Your Sneakers. “When you don’t eat enough food, your body goes into ‘rebuild mode,’ and that’s not productive for a workout.” If your body's low on fuel, you might experience things like hunger pangs, dehydration, and dizziness during exercise—none of which will make your workout very rewarding.

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“We need to remove as many barriers as possible to any kind of exercise and make it somewhat enjoyable,” Lansing says.  This is especially true now—after a stressful, largely sedentary year in COVID-19 quarantine—when people might be struggling to get back into a fitness routine. “You need to have fuel in your system so that you’re not suffering through a workout.”

What happens to our bodies when we work out on an empty stomach?

Like Lansing said, for short, low-intensity workouts, working out on an empty stomach isn't the end of the world—as long as you don't make it a habit. But for moderate or high-intensity workouts that last an hour or longer? You need to fill the tank.

Natalie Rizzo, MS, RD, sports dietitian and founder of Greenletes, echoes Lansing’s sentiment that if our body is an engine, then food is gasoline. When we eat carbohydrates, the body stores glycogen in the muscles and liver, which is the primary energy source for 30-60 minutes’ worth of exercise. When we don’t eat, we don’t have enough glycogen to tap into, and our bodies will look for other places to find the energy they need. That's where things get tricky.

“Your body may burn fat or muscle as fuel. Although this sounds like a good thing, using fat as fuel is an inefficient process that’s taxing on the body,” says Rizzo. This can cause problems immediately, like fatigue, dizziness, and low blood sugar. But after a while, the implications can add up, putting the body at risk of vitamin deficiencies, mood fluctuations, and lowered immunity. Plus, it’s counterproductive to work out on an empty stomach: “Trying to force the body to use fat as fuel can negatively affect your performance,” explained Rizzo. “This means you won’t be able to work out as hard and may even feel like you’re losing your fitness level.”

Now, I’m getting hungry… when and what should I eat before my workout?

Amy Gorin, MS, RD, a plant-based dietitian and owner of Amy Gorin Nutrition, explains that foods that are high in protein and carbohydrates are your best bet and should be eaten one to three hours before a workout. "The carbohydrates help fuel a workout and maximize athletic performance,” says Gorin. As for the protein? “This is so that amino acids are made available to your muscles, and so your body doesn’t break down your muscles for protein," says the dietitian.

If you want to get a little more particular about your nutrition and meal-prep, Rizzo recommends using this IG graphic as a general rule of thumb: “The longer out you are from a workout, the more complex the meal,” she says. “If you’re eating two hours before a workout, eat a light meal with carbs, protein, and fat, like a rice bowl or an egg sandwich. If it’s 60 minutes before a workout, focus on carbs with just a little bit of protein, like toast with nut butter. And If you only have 30 minutes until your workout, eat something that is made up of easy-to-digest carbs, like fruit or an energy bite." Looks like I'll be stocking up on those A.M. granola bars from now on.