Why you shouldn’t work out on an empty stomach

2022-12-19

Find out how working out on an empty stomach could impact your performance and recovery — and when to fuel up before your training session.

As you head out the door for a 6 a.m. HIIT workout or hop on your bike for a sunrise Peloton class, the thought of wolfing down a full-fledged breakfast may leave you feeling seriously disturbed. So instead of devouring a bowl of cereal, you might tackle your workout with your tummy empty and growling.

But is working out on an empty stomach actually NBD? “I definitely do not recommend it,” says Abby Chan, M.S., R.D.N., a registered dietitian nutritionist and the co-owner of EVOLVE Flagstaff in Arizona. “Your body will always do better in a fed state, no matter what.”

Ahead, Chan breaks down why fueling up ahead of a training session is so important, as well as the potential risks of working out on an empty stomach. Trust, munching on a slice of toast before your run is well worth it.

The Problem with Working Out On an Empty Stomach

First things first, a quick bio lesson. During exercise, your body taps into stored forms of carbohydrates, known as glycogen, in your liver and muscles for energy, according to the American College of Sports Medicine (ACSM). Your liver will break down its glycogen as you sweat to maintain your blood glucose levels, which your muscles will use for energy in addition to their own glycogen stores, according to the Cleveland Clinic. Your body can store roughly enough glycogen to support you through a moderate-intensity or short-duration workout, according to the ACSM. When this glycogen is nearly used up, your body may turn to fatty acids for fuel, according to the Mayo Clinic.

Some folks believe that by working out on an empty stomach, your body will break down those fatty acids sooner via fat oxidation and thus encourage “fat burning,” says Chan. Some studies have found that exercising at a low-to-moderate intensity in a fasted state generally does promotehigher levels of fat oxidation during your workout than exercising after eating carbohydrates. But evidence supporting this effect following long-term fasted training is lacking, and the uptick in fat oxidation diminishes as workout intensity increases, according to a 2020 review published in Nutrients. What’s more, the majority of the participants in these studies were trained males between 20 to 30 years old, according to the review, so the results may not apply to the general population. And with some types of cardio, your body may break down muscle — not fat stores — for energy, as Shape previously reported.Translation: Currently, it’s tough to say whether you'll experience any significant fat-oxidation effect from working out on an empty stomach.

Skipping your pre-workout meal or snack can lead to short- and long-term health repercussions too. Some folks may experience nausea, fatigue, and discomfort while working out on an empty stomach, according to the ACSM. If your workout is only 20 minutes and low-intensity (say, a yoga class), having a pre-workout meal or munchie may not affect your training session too much, says Chan. However, “especially if someone's working out longer than 45 minutes or an hour, [eating beforehand] will actually allow your body to have more energy and have more power and stamina throughout that workout,” she adds.

And if you’re training regularly, regardless of the activity, not fueling up properly can affect your ability to recover, says Chan. When you begin exercising in a fasted state, your blood sugar is likely already low, so your body will utilize the glycogen on hand to help raise your glucose levels and effectively support your brain and organ function, explains Chan. That means your body will have less glycogen available to mobilize to carry you through your tough workout and recovery period.

“You’re kind of running on an empty tank,” she adds. “If your muscles don't have enough fuel or don't have full glycogen stores, that's going to inhibit and decrease recovery in the long run.” In fact, research shows the longer and more intense your activity, the greater reduction of glycogen stores — and thus the longer it’ll take to replenish them and for your body to fully recover.

The TL;DR: You shouldn’t let working out on an empty stomach become a habit. “The reason why you're working out most likely is to start to improve muscle function and feel like you can push hard,” says Chan. “But if you're not in a fed state, most likely your muscles are gonna feel more fatigued… and if you don't have the energy to work out or recover, then you’re not going to be able to show up day in and day out.”

When to Eat Before a Workout

Generally speaking, you’ll want to eat a meal within 90 minutes to an hour ahead of your workout, notes Chan. And that means heading straight to the gym after eight hours of beauty rest isn’t ideal.

But if cooking and eating a full plate is out of the cards, whether it be due to scheduling conflicts, time requirements, or your personal preferences, at least have a carb-rich snack 30 minutes beforehand to help maintain your blood sugar levels, suggests Chan. “It can be something super small — a banana, a fruit strip, apple sauce, or a piece of toast,” she adds. “If you haven’t eaten within the last two hours, you're not going to fully go into a deficit and die, but you'll probably have a better workout if you've had something small.”

Energy boost aside, fueling up ahead of your lifting session or Pilates class will ensure you don’t feel ravenous after your training session, says Chan. While the risk of inhibiting recovery should be enough to convince you to work out with your stomach full and satisfied, the possibility of feeling hangry later surely seals the deal.

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