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Fasted Cardio May Have More Drawbacks Than Benefits, Research Shows

Runner’s World 2/3/2022 Ashley Mateo

If you’ve ever woken up at 5 a.m. to squeeze in a morning run and haven’t scarfed down a banana, energy bar, or any other food on your way out the door, then you’ve already done fasted cardio. Sometimes time is of the essence, and there simply isn’t an opportunity to fuel and digest before a run—or sometimes, you just can’t stomach the calories that early.

You may have also heard, though, that some people link fasted cardio to weight loss and performance benefits. So, should running on empty always be your morning ritual? Here’s what the experts have to say about completing cardio workouts in a fasted state.

What is fasted cardio?

Fasted cardio is quite simple: “It literally just means doing a workout after not eating for some amount of time,” explains Lauren Antonucci, R.D.N., a board-certified specialist in sports dietetics based in New York.

Some athletes may claim they’re doing fasted cardio by skipping lunch and heading straight from the office to a run, but the scientific literature specifies that the body needs a 10- to 14-hour period of not eating to be truly fasted, adds Meghann Featherstone, R.D., a Cleveland-based board-certified specialist in sports dietetics. So, for most people, fasted cardio would occur first thing in the morning.

What are the potential benefits of fasted cardio?

Once you’ve been in a fasted state for a while, your glycogen stores will be slightly depleted, and your body will look for alternative fuel sources. “Within a few minutes of starting a fasted workout—no matter how far you’re planning on running—the percentage of fat you would burn in that workout would be a little bit higher,” says Antonucci.

“If we run fasted, we tap into our fat stores as a fuel source sooner, so we’re running more on oxidized fat versus glycogen or carbohydrates,” says Featherstone.

Some research helps to demonstrate this source of fuel change that happens during fasted cardio. For example, a systematic review and meta-analysis published in 2016 in the British Journal of Nutrition states that an aerobic workout performed in a fasted state leads to greater fat oxidation, compared to a cardio workout done after eating.

More specifically and more recently, a study published in the International Journal of Environmental Research and Public Health in 2022, involving 12 male participants, found that fasting for six hours and then running on a treadmill for 20 minutes at a low to moderate intensity (45% to 65% of VO2 max) enhanced fat metabolism, when compared to running in a fed state.

What are the drawbacks of fasted cardio?

People tend to latch on to those concepts, and see fasted cardio as a path to weight loss (you’ll burn more fat!) or enhanced performance (if you can tap into fat stores for long periods, you’ll never hit the wall!).

When it comes to weight loss, all that fat-burning potential from fasted cardio does sound appealing. But “you’ll still be burning the same amount of calories,” says Antonucci. If you’re eating within the nutritional budget determined by your training plan and weight loss goals, “burning a little bit higher percentage of fat is not really that important to your body weight over time or your body composition,” she says.

And remember, your body doesn’t automatically burn fat instead of glucose in a fasted state; it may turn to protein instead. “Research shows that there’s an increase in muscle breakdown when we exercise fasted, so it could actually decrease our strength,” says Featherstone. Running in a fasted state yielded twice the amount of protein breakdown in muscles than in a non-fasted state, according to a study published in the Strength and Conditioning Journal.

Let’s not forget those endurance benefits… People were actually able to perform aerobically for longer after eating than when they fasted, a more recent meta-analysis published in the Scandinavian Journal of Medicine & Science in Sports found. And additional research shows that fasting has negative impacts on the intensity and volume of training—both of which can hinder your performance. “Intensity increases with our runs, and with that our dependence on carbs increases,” says Featherstone. “If we don’t have carbs to pull from, the intensity of our workout is going to suffer.”

Any time you do any high-intensity workout—intervals, fartleks, any speedwork—you’re going to burn a super high percentage of carbs no matter what. “Not fueling beforehand just shortchanges your energy and ability to work hard,” says Antonucci. Plus, your rate of perceived exertion is much higher at a lower intensity when you’re running off fat versus carbs, says Featherstone, and you’re likely to finish fasted high-intensity runs feeling terrible or hit a wall sooner in longer efforts.

So, is fasted cardio right for you?

If you’d rather use your prerun time in the morning to sleep in a little bit or you can’t handle the idea of eating early in the morning, sure, it’s okay to do fasted cardio. “Just make sure you’re only doing easy runs when you’re in a fasted state,” says Featherstone. Because of limited glycogen, don’t regularly plan long (90 or more minutes) or hard workouts when fasted.

Otherwise, when it comes to really putting in the work, it’s better to run within one to three hours after eating breakfast. “Running isn’t just about burning fat,” says Antonucci. “When you’re properly fueled, it’s going to feel better mentally and physically.”

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