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What Is Fasted Cardio & Is It Beneficial?

Intermittent fasting has become an increasingly popular way of eating for its long list of health benefits, including weight management and a decrease in disease risk1. But are there any benefits to fasting before a workout?

To find out what fasted cardio entails and whether it's safe and effective, mbg spoke with experts in exercise physiology and sports dietetics, who shared insight about exercising on an empty stomach. 

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What is fasted cardio?

The term fasted cardio simply means doing cardiorespiratory exercise in a fasted state. This often applies to people who wake up and work out before eating breakfast. It can also mean someone works out while in a period of fasting.

"Generally, it takes about four to five hours to completely digest a meal," exercise physiologist John Ivy, Ph.D., says. "So after a meal, one would have to wait probably a minimum of four hours to get the benefit of fasted cardio." 

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Are there benefits to fasted cardio? 

When exercising in a fasted state, the body has to rely on fat storage for energy, Ivy explains. So many people believe fasted cardio could help decrease body fat more efficiently. However, registered dietitian and author of The Sports Nutrition Guidebook Nancy Clark, M.S., R.D., CSSD, says that's not necessarily true.  

Running on empty may cause a person to burn 300 calories of fat, Clark says, but if they come home and eat an 800-calorie meal, they haven't really lost anything. "Burning fat does not equate to losing body fat," she tells mbg. 

Exercising first thing in the morning does, however, take advantage of certain hormonal changes that occur during the early hours2, Ivy says. Things like low insulin levels and elevated testosterone levels can help the body burn stored fat, he explains. 

Elite athletes may engage in fasted cardio a few times a week to build mental toughness and enhance their fat burning capacity, Clark says, but it's not necessary or all that beneficial for a recreational athlete. 

Are there side effects to fasted cardio?

"Exercising in a fasted state is harder physically and mentally," Ivy says. "You do not have the rapidly available fuel stores for strenuous exercise such as blood glucose, which after an overnight fast could be quite low." Because of this, people who are exercising after a fast should stick with low to moderate exercise, not high-intensity workouts.

"The brain needs energy to focus, and if the brain isn't fed, you can lose your ability to concentrate," Clark says. This can lead to accidents and injuries. People should listen to their bodies and stop or slow down if they begin to feel lightheaded, shaky, or dizzy while working out. 

It's also important to keep hydration levels in mind. "The body is usually not well hydrated when first getting up from a night's sleep," Ivy tells mbg. Before exercising, he recommends drinking water, tea, or another low-calorie beverage. Coffee might not be the most hydrating, but it can have some benefits on exercise performance

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Who should try fasted cardio? 

Whether or not a person should try fasted cardio depends on their personal exercise goals.  

For those trying to burn fat and lose weight, fasted cardio can be effective. However, it may not be any more effective than exercising while fed. In fact, one study conducted on a fasted and a non-fasted group found that both groups experienced a similar loss in weight and fat mass3. "This may be due to the fact that one can exercise longer and more intensely in a fed state and burn more calories," Ivy says.

Anyone trying to build muscle will want to avoid it, Clark says. "When you're fasted, your body is in muscle breakdown mode and that's sort of counterproductive to what you as an athlete want to do," she explains. 

Additionally, people with eating disorders or in recovery from eating disorders should avoid fasted cardio, says Clark.

Bottom line.

Fasted cardio can lead to fat burn and weight loss but not necessarily at a higher rate than other exercise methods. For the most part, it's safe as long as the person doing it is engaging in low to moderate exercise and staying properly hydrated. "Also, it is best to ease into a fasted cardio program," Ivy says. Over time, the body will adapt and the training should become easier. If they don't, it's a good idea to go back to non-fasted exercise. 

After finishing any workout, Clark says it's important to refuel with carbohydrates and protein to build and repair muscles.


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