Do you need to eat before your morning workout?

The internet is full of people with opinions on the best way to optimize a workout. Fasted cardio, fueling before a workout, or somewhere in between—which is the best option?

Because diet and fitness trends seem to constantly change, it can be hard to stay on top of the latest science versus fads from social media perpetuated by people who are only speaking anecdotally or based on their hearsay.

Here’s how not eating before a workout impacts the body, who may want to try fasted cardio, and what you might want to eat before a workout if you’ve decided that works best for you.

Does Fasted Cardio Burn More Fat?

Not eating before a workout falls is considered “fasted cardio.”

Fasted cardio is simply performing aerobic exercise without having consumed any food (especially carbs) for at least 8–12 hours prior, Dana Ellis Hunnes, PhD, MPH, RD, the senior clinical dietitian at UCLA Medical Center, assistant professor at UCLA Fielding School of Public Health, and author of Recipe for Survival, told Health.

Even if you’re not intentionally doing your cardio in a fasted state, morning workouts without breakfast almost always fall into this category.

“It’s typically done first thing in the morning, as this is when the body is always in a fasted state, but it can be done at just about any time of the day when you haven’t eaten in a few hours,” explained Jen Roper, PhD, CSCS, a certified sports nutritionist and associate professor of Health and Human Sciences at Loyola Marymount University.

Working out on an empty stomach or without eating beforehand can affect the primary fuel your muscles burn for energy during your workout.

The primary fuel source for higher intensities of exercise is often glucose—the simple sugar that comes from the breakdown of carbohydrates.

Bodies store a certain amount of excess carbohydrates in a storage molecule known as glycogen. The body can then tap into glycogen stores and break down larger carbohydrate molecules into usable glucose that muscles and other tissues of the body need energy during exercise.

Roper explained that, by nature, when you are in a fasted state, your blood sugar is low.

“This means that the body will begin to break down stored glycogen (stored glucose) quicker to supply the body with energy for the workout,” she said. “When the glycogen is depleted, your body will turn to fat as the main source of fuel,” said Roper.

That’s why it’s often thought that fasted cardio may boost fat loss, though current research on that is mixed.

“While this is thought to be helpful for fat loss, the research is mixed as to the full effectiveness [because] the biochemical processes are a bit more complex than that,” said Uma Naidoo, MD, a Harvard-trained nutritional psychiatrist, director of Nutritional and Metabolic Psychiatry at Massachusetts General Hospital, and author of the national and international bestseller This is Your Brain on Food and the upcoming book Calm Your Mind With Food.

Roper agreed that the research has been inconclusive about whether exercising on an empty stomach actually burns more fat.

“It’s also important to note that exercising at a moderate-to-high intensity in a fasted state may be difficult, and results in reduced overall calorie burn. So, it potentially can work for some, but the literature doesn’t support this tactic for all,” she said.

Like many fitness-related topics, results often vary from person to person.

A Potential Downside of Fasted Cardio

While there is some evidence in support of fasted cardio accelerating fat burning, there is some conflicting evidence that indicates not eating before a workout can increase cortisol and inflammation, both of which can impede fat loss and compromise health.

“One issue that can happen is that the body starts to turn to a process called gluconeogenesis, breaking down protein for fuel in the absence of eating prior to working out,” said Naidoo.

A 2015 study found that exercise following an overnight fast was more effective at reducing body fat compared to non-fasted exercise, but participants who fasted before exercise experienced increased cortisol levels that could negatively impact long-term weight loss.2

According to Hunnes, exercise in and of itself can be stressful to the body, independent of your fueling status.

“[Exercise] can temporarily lead to increased cortisol (if it’s high-intensity exercise), and stress on the muscles, but these are good and positive stressors that dissipate quickly after the workout and actually benefit health significantly,” said Hunnes.

While it’s plausible that exercise in and of itself is enough to spike the body’s cortisol levels, Roper noted that the research on fasted cardio’s ability to do the same is not clear.

That said, a lack of consistency in the research doesn’t mean there aren’t things to be cautious about in terms of fasted cardio stressing the body, said Roper.

Is Working Out On an Empty Stomach Bad for You?

According to Roper, fasted exercise is generally safe for most people, but it may lead to side effects.

“Because blood glucose gets depleted during fasting cardio, you can experience some side effects including lightheadedness and dizziness,” she said. “If it is prolonged cardio, you could potentially pass out, so if you commonly experience side effects from low blood glucose, you may want to avoid fasted cardio.”

Hunnes and Naidoo agreed and explained that unless you have a certain medical condition, like diabetes or reactive hypoglycemia, fasted exercise is most likely safe.

“Someone with type 2 diabetes may not do well with fasted cardio first thing when they wake up, as this may heighten insulin resistance,” Naidoo said. “With someone who is taking insulin for the treatment of diabetes management, their situation may be different as well.”

Who May Benefit From Fasted Exercise

Choosing whether you should eat before a workout can be an individual decision but it also may be a medical decision, in certain cases.

Hunnes explained that children should also have a small meal or snack before harder physical activity because they have a reduced capacity to store glycogen/energy for exercise, and more energy is going towards growth.

For adults, the decision is often based on personal preference, as well as the length and intensity of the planned workout, Roper explained.

“Moderate-to-high intensity workouts and prolonged endurance workouts (more than 1.5 hours) are going to benefit more from eating beforehand,” she said. “It’s difficult to maintain higher intensities once glycogen and blood glucose are depleted, which happens quicker when fasted.”

The other factor you should consider when deciding if you should eat before a workout is your primary fitness goal.

“If you’re exercising for fitness and weight loss, you may want to adjust the timing and what you eat before and after exercising,” said Hunnes. “If you’re training for an event, you may also need to change what, when, and how you eat.”

Hunnes also explained that it’s not only whether or not you eat before a workout that matters—what you eat after a workout plays a key role in helping your muscles and body as a whole recover from your exercise session.

“Once you’ve done damage to the muscle (lifting weights) or worked out the muscles (cardio), you want to replenish the carbohydrate (glucose) stores you’ve depleted and help muscle repair with healthy anti-inflammatory proteins,” she said.

What Experts Recommend You Eat Before a Workout

If you do decide to eat before your workout, there are a few things to consider.

“Slow-burning carbohydrates like oats, whole wheat toast, and whole fruit are popular pre-workout foods,” said Naidoo. “The goal is to ensure that you are not feeling too full to get a good workout in, but also that the glucose is still circulating in the system.”

In order to ensure you aren’t too full to work, Naidoo recommends eating 2–3 hours before your workout. If you’re eating 1–2 hours beforehand, opt for a lighter meal.

Hunnes recommends something light and full of carbs, with small amounts of fat and protein, like half a bagel with peanut butter, or a banana with a handful of nuts.

She notes these recommendations apply to healthy individuals without metabolic conditions.

“If you have diabetes or other medical condition that [makes it so] you should eat before, then you should get guidance from your endocrinologist, cardiologist, dietitian, or another medical provider based on your specific needs,” she said.

Roper explained that if you do eat closer to the time of your workout, liquids, and carbs that are not high in fiber or fructose are the best option.

But ultimately, experts agree that the logistics of what you eat and when you eat are personal. What matters most is that people move regularly.

“Whether your cardio is fasted or fed, just do it,” said Roper. “Just remember, most things are a personal preference, and what may work for one person may not work for all.”