The idea of gorging on a full breakfast may definitely bother you as you leave the house for a 6 a.m. HIIT workout or mount your bike for a daybreak Peloton class. So, rather than indulging in a bowl of cereal, you might start your workout with an empty stomach and a growling stomach.
But is exercising on an empty stomach really not that bad? Abby Chan, M.S., R.D.N., a registered dietitian nutritionist and the co-owner of EVOLVE Flagstaff in Arizona, adds, “I definitely do not endorse it.” No matter what, your body will always function better when it is fed.
Chan discusses the importance of refueling before a workout as well as the possible risks of working out in the following. Trust, munching on a slice of toast before your run is well worth it.
The Problem with Working Out On an Empty Stomach
Let’s start with a little bio lesson. According to the American College of Sports Medicine, your body uses glycogen, or carbohydrate forms that have been stored in your muscles and liver, to provide energy while you are exercising (ACSM). According to the Cleveland Clinic, your liver will break down its glycogen as you perspire to maintain your blood glucose levels, which your muscles will consume for energy in addition to their own glycogen stores. According to the ACSM, your body has roughly enough glycogen reserves to get you through a short or moderate-intensity workout. The Mayo Clinic states that when this glycogen is almost depleted, your body may start using fatty acids as fuel.
Some people think that by exercising on an empty stomach, your body will metabolize those fatty acids more quickly and promote “fat burning,” according to Chan. According to several research, working exercise at a low to moderate intensity while fasting generally leads to higher levels of fat burning than working out after consuming carbs. A 2020 review published in Nutrients found that the boost in fat oxidation declines as workout intensity increases and that there is little data to support this impact after long-term fasted training. The study also notes that the bulk of the participants in these research were trained males between the ages of 20 and 30, therefore the findings might not be generalizable. Additionally, as previously revealed by Shape, your body may burn muscle rather than fat stores during specific forms of exercising. Currently, it’s difficult to predict whether exercising on an empty stomach will have a major impact on fat oxidation.
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.