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Should You Fast Before Your Next Cardio Workout?

Having a bite before you bike, run, or swim can affect how you feel and perform.

By Jessica MigalaJune 9, 2023

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To eat—or not to eat. If you’re getting ready for a workout, that’s the question. Some exercisers have their routine down, nibbling on a banana or mini pb+j before they clip in for a ride or head out for a few miles, while others purposefully skip a bite. Whether or not you squeeze in a snack (or meal) ahead of time may impact your performance, motivation, appetite, and more. Here’s what you need to know about fasted cardio, plus tips for deciding if it’s right for you and your fitness and health goals—and how to do it right.

What is Fasted Cardio?

Fasted cardio is simply exercising on an empty stomach. “Fasted cardio refers to doing a cardio workout before eating anything in the morning,” says Sarah Schlichter, MPH, RDN, nutrition consultant and owner of Nutrition for Running and Bucket List Tummy. If you’re practicing intermittent fasting (where you have defined periods of fasting, or no eating, followed by feeding), then a fasted cardio session would come at least 6 to 8 hours after eating, she explains. 

Some people will consume coffee or a pre-workout before a fasted workout for an energy kick to get going. (A “pre-workout” is a drink mix that contains certain ingredients to boost workout performance.) Whether or not this is a true fasted workout is up for debate. “There are no clear-cut rules,” for fasted workouts, says Schlichter. 

Are There Benefits of Fasted Cardio?

Some people find fasted cardio easier on their digestive system. And that comfort during a workout can make it worth skipping a pre-cardio meal. Outside of that, there may also be body composition benefits to going out on an empty stomach:

Greater Fat Loss (Possibly)

One of the biggest claims around fasted cardio has to do with improved fat loss, but does fasted cardio burn more fat? “Some research indicates that fasted cardio can lead to fat loss,” explains Schlichter. Without food that morning, your body doesn’t have available glucose, which is its go-to source of fuel to give you that zip during cardio. Still, your body needs to pull fuel from somewhere, so it will tap into fat stores instead. The result is that you burn a greater percentage of calories from fat as opposed to carbohydrates. But it’s the bigger picture that matters most. This difference is minimal when taken in the context of your overall diet, Schlichter says. If your goal is weight loss, it’s more important to maintain a calorie deficit, which can come from eating fewer or burning more calories through activity, or a combination of the two. 

May Improve Insulin Function

Insulin is the hormone that helps push blood sugar into your cells for energy. Problems with insulin function play a role in the development of type 2 diabetes, according to the Centers for Disease Control and Prevention (CDC). There is some evidence that fasted cardio may help reduce fasting insulin levels better than non-fasted cardio, according to a six week study of aerobic exercise in adult men who were overweight or had obesity, published in the Journal of Exercise Science & Fitness in 2023. It’s important to keep in mind though that weight, BMI, body fat percentage, waist and hip circumference, and cholesterol decreased in both the fasted and eating groups. What does that tell you? If you’re looking to improve body composition, both approaches can help you get there.

May Help You Eat Fewer Calories 

If weight loss is your goal, then fasted exercise may help you reduce calories more so than a pre-exercise meal or snack. One study in Human Kinetics Journal in 2022 compared the effects of 45 minutes of steady-state cycling in the evening after a seven-hour fast compared to exercising two hours after eating. The researchers found that though men tended to eat more food during a post-exercise meal after fasted exercise (an effect not seen in women), people ate about 443 fewer total calories over the course of the day. (Why? Fasting made them skip a meal.) The researchers also note that it’s possible people could have made up for those calories the next day. 

That said, in the study, people had less motivation and energy to exercise when fasted—and they said they enjoyed exercise less. Their performance dipped by about four percent when fasted versus eating first. (Keep reading for more of the potential downsides of fasted cardio.) 

Overall, though, a review in the Proceedings of the Nutrition Society in 2019 concluded that more research, particularly long-term research, is needed to determine how fasted versus fed exercise affects one’s health overall, and if there are certain people who would benefit from the approach more than others.

Downsides to Fasted Cardio

Honestly, some people might not feel that great if they do fasted cardio workouts. According to Schlichter, you may experience:

  • Dizziness

  • Lightheadedness

  • Fainting

These symptoms are mainly due to the effects of low blood sugar caused by going so long without eating. If you don’t feel your best or most energetic during a fasted cardio workout, it’s also likely your performance will suffer—especially in longer workouts.  Schlichter points to a review and meta-analysis of 46 studies published in the Scandinavian Journal of Medicine and Science in Sport, which found that eating before exercise was beneficial for performance for aerobic workouts lasting more than an hour; though fasting before shorter workouts wasn’t found to be detrimental. 

Also, it’s important to really assess your motivations for fasted cardio. Then, reflect on if it is the best choice that supports your health. If you skip a meal in order to do fasted cardio and eat hours later, it’s possible you could wind up under-fueling your body, which can have ramifications down the line: “Chronic undereating while exercising can lead to muscle and bone loss, disrupted hormones, overtraining, and burnout,” Schlichter explains. 

What to Know Before Trying Fasted Cardio

If you prefer to do fasted cardio—say, your stomach just feels better when you haven’t eaten anything before exercise—then there are steps you can take to do so safely. Here’s what Schlichter recommends:

Save it for Lighter Workouts

Going into a HIIT session, long run, or speed workout fasted is a recipe for disaster. “Save fasted cardio for a shorter morning workout or a recovery workout,” she explains. For example: Go out for a light, slow run for a couple of miles or try a recovery ride or walk. Stick to workouts that are 30 minutes or less, suggests Schlichter.

Try a Sports Drink

If your reason for skipping eating before a workout is because your stomach feels heavy and sensitive when you do, consider sipping a sports drink, she recommends. A sports drink supplies some calories and carbohydrates for fuel, but the liquid is quicker and easier to digest compared to solid food. This would not be fasted cardio because you’d be taking in calories, but it could be one solution to give your body fuel—comfortably. 

Eat a Bedtime Snack

Have a run or ride on your schedule for the early morning? Schlichter recommends eating a balanced nighttime snack, which will help top off your body’s glucose (energy) stores as much as possible prior to your workout. Cereal, nuts and cheese, fruit and yogurt, or a piece of whole grain toast with a smear of nut butter can do the trick.

Refuel After

Eating something after your workout jumpstarts your body’s recovery process. Not sure what to eat after fasted cardio? Consuming carbohydrates replenishes your glycogen (carb) stores, while protein facilitates muscle repair. Keep in mind that if you practice intermittent fasting and therefore do a cardio workout early in the morning and wait to break your fast around 11 or noon, you may not only feel uncomfortably hungry, but also miss out on the important post-exercise window to refuel, which can stymie your progress. 

Other Types of Exercise While Fasting

Most of what we’ve talked about here also applies to doing strength workouts fasted. If you’re doing a short resistance training session (less than 30 minutes), you should be okay, says Schlichter. But high-intensity, interval-based, or long-duration strength workouts really do require eating something ahead of time to help you feel your best. “Plus, fuel before and after will enhance muscle building, which is usually a goal for those who are strength training, while fasting will break down muscle protein quicker,” she explains.

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