We ask experts for their thoughts on fasted cardio.

Has anyone ever suggested you work out on an empty stomach? Doing cardio before or without fueling with food, otherwise known as fasted cardio, is a hot topic in the fitness and nutrition world.

Like many health trends, there are fans and skeptics. Some people swear by it as a quick and effective way to lose fat, while others believe it’s a waste of time and energy.

Fasted cardio doesn’t necessarily mean you’re sticking to an intermittent fasting routine. It could be as simple as going for a run first thing in the morning, then eating breakfast after.

We talked with three fitness and nutrition experts about the pros and cons of fasted cardio. Here’s what they had to say.

Hitting the treadmill or upright bike for a cardio session before eating is popular in weight loss and fitness circles. The possibility of burning more fat is often the main motivator. But how does that work?

“Not having excess calories or fuel on hand from a recent meal or pre-workout snack forces your body to rely on stored fuel, which happens to be glycogen and stored fat,” explains Emmie Satrazemis, RD, CSSD, a board-certified sports nutritionist and nutrition director at Trifecta.

She points to a few small studies that suggest working out in the morning after 8 to 12 hours of fasting during sleep may allow you to burn up to 20 percent more fat. However, there are also studies showing that it makes no difference in overall fat loss.

But know that there’s a difference between adding muscle mass and preserving muscle mass.

“As long as you’re eating adequate protein and continuing to use your muscles, research suggests that muscle mass is pretty well protected, even in an overall calorie deficit,” explains Satrazemis.

That’s because, when your body’s looking for fuel, amino acids aren’t as desirable as stored carbs and fat. However, Satrazemis says your supply of quick energy is limited, and training too hard for too long while fasting is going to cause you to run out of gas or potentially start breaking down more muscle.

In addition, she says that eating after a workout allows you to replenish these stores and repair any muscle breakdown that occurred during your workout.

This reason may seem like a no-brainer, but it’s not uncommon to question why we do something, even if it makes you feel good. That’s why Satrazemis says the decision to try fasted cardio comes down to personal preference. “Some people just prefer to work out on an empty stomach while others perform better with food,” she says.

If you plan on doing an activity that demands high levels of power or speed, you should consider eating before performing these workouts, according to David Chesworth, an ACSM-certified personal trainer.

He explains that glucose, which is the quickest form of energy, is the optimal fuel source for power and speed activities. “In a fasted state, the physiology doesn’t typically have the optimal resources for this type of exercise,” Chesworth says. Therefore, if your goal is to become fast and powerful, he says to make sure to train after you’ve eaten.

Sitting down to a meal or even a snack prior to doing cardio can make you feel sick during your workout. “This can especially be the case in the morning and with high fat and high fiber foods,” explains Satrazemis.

If you can’t handle a larger meal or you don’t have at least two hours to digest what you eat, you may be better off consuming something with a quick source of energy — or performing cardio in a fasted state.

To do cardio in a fasted state requires you to be in excellent health. Satrazemis says you also need to take into consideration health conditions that may cause dizziness from low blood pressure or low blood sugar, which could put you at a greater risk for injury.

If you decide to try out fasted cardio, follow a few rules to stay safe:

  • Don’t exceed 60 minutes of cardio without eating.
  • Choose moderate- to low-intensity workouts.
  • Fasted cardio includes drinking water — so stay hydrated.
  • Keep in mind overall lifestyle, especially nutrition, plays a bigger role in weight gain or loss than the timing of your workouts.

Listen to your body and do what feels best to you. If you have questions about whether or not you should do fasted cardio, consider consulting a registered dietician, personal trainer, or doctor for guidance.

Sara Lindberg, BS, MEd, is a freelance health and fitness writer. She holds a bachelor’s degree in exercise science and a master’s degree in counseling. She’s spent her life educating people on the importance of health, wellness, mindset, and mental health. She specializes in the mind-body connection, with a focus on how our mental and emotional well-being impact our physical fitness and health.