Cue stomach growls.

By Susan Brickell
April 04, 2019

The idea of waking up crazy early, skipping breakfast, and dragging our butts to the gym or spin studio is enough to make us want to hit the "snooze" button, throw the duvet over our heads, and Seamless an egg sandwich to our beds.

Believe it or not, there are many people who swear by early workouts without fueling up first, because they believe that "fasted" workouts are better for you and could actually burn more fat.

Heading up the "fasted cardio" club? Jennifer Lopez and Alex Rodriguez, the fittest couple in Hollywood, of course.

"Another beautiful day in New York City, about to go do a little fasted cardio," A-Rod said in his Instagram Story on Wednesday, walking alongside J.Lo. "That means cardio before breakfast. This is from our trainer, Dodd." ("Hi, Dodd!" J.Lo playfully adds.)

Instagram

But is fasted cardio really a good idea? We asked expert trainers to weigh in on if it's safe to exercise on an empty stomach, and whether or not you should. Here’s what you need to know.

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What is fasted cardio though?

Fasted cardio is cardiovascular training performed when the digestive system is no longer processing food, which can happen four to six hours after eating or in the morning after you've woken up, explains Vince Sant, co-founder and lead trainer for VShred.com. In other words, it's doing cardio without any food in your body.

People who practice intermittent fasting have a schedule that is more accommodating to fasted training, since they're restricting calorie intake for a few hours or don't consume any food at all for a full day or longer, which by the way is not totally recommended by nutritionists. For most, early morning workouts after your body has been fasting during sleep is the best way to experiment with fasted cardio, says Sant.

The benefits of fasted cardio

Fasted cardio might help you burn more fat throughout the day, trainer Danielle Natoni tells Health. The idea is that when you work out when you haven't eaten recently, you are burning fat—aka stored energy—versus the energy from the food that you just consumed, she explains.

Fasted cardio is actually Natoni's preferred method of exercise. "On the rare occasions where I find I have to exercise no longer fasted, I feel slower and more sluggish," she says.

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Wait, will you actually burn more fat faster?

Maybe. Small studies have suggested that cardio in a fasted state can burn up to 20% more fat. But other research suggests that the difference between fasted and fed training is minimal as it relates to calories burned or fat loss, Sant notes. It really becomes a question of what is sustainable for you, he says. And what your goals are: Fed training may improve exercise performance, according to research comparing fed and fasted workouts.

Is fasted cardio safe?

As long as it's not taken to an extreme, fasted cardio is safe. Just keep in mind that your body needs fuel to function, so an intense, two-hour cardio session after eight hours of sleep might cause side effects of low blood sugar like lightheadedness. That could pose a serious safety risk when running alongside traffic or using heavy equipment at the gym, Sant says. You might also be more likely to get dehydrated during fasted training.

Should you try fasted cardio?

To see if fasted cardio is for you, start out small. First try eating something light like a banana or a piece of toast pre-workout and see how you feel, Natoni suggests. If you feel good, next time you can try exercising right after waking up in the morning. Just be sure you consume enough water before working out and do light to moderate work instead of something really intense, she adds.

Remember that every body is different, so whether you plan on working out fasted or fed is up to you and what works for your body. And ultimately, the key to weight loss is maintaining an overall caloric deficit through a well-balanced program encompassing diet and exercise, Sant says. In the grand scheme of things, when you eat probably has less to do with weight and fat loss than what you are eating and how much effort you are putting into your fitness regimen—not to mention that your genes, metabolism, and age also influence your weight, he points out.

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