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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.
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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, even for just 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.
Also, your body may not automatically burn fat instead of glucose in a fasted state; it may turn to protein. “Research shows that there’s an increase in muscle breakdown when we exercise fasted, so it could actually decrease our strength,” says Featherstone.
While the research on protein metabolism in a fasted state is limited, one review article published in the Strength and Conditioning Journal in 2011 states that exercising in a fasted state yielded greater protein loss than in a non-fasted state, suggesting it might not be best for those looking to build muscle. Another scientific review published in 2020 states that while protein doesn’t provide much fuel for exercise on a regular basis, when you restrict calories, that contribution markedly increases.
Let’s not forget those endurance benefits: People were actually able to perform aerobically for longer after eating than when they fasted, a meta-analysis published in 2018 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.
Finally, while there is some research on the benefits of fasted cardio, there’s a need for more long-term, bigger studies and those done on different populations. A review paper published in the Proceedings of the Nutrition Society in 2018, points out that while fasted cardio enhances fat metabolism, we still need to know how that might affect those with cardio-metabolic conditions and how training in a fasted state might affect outcomes when doing other types of workouts, like resistance training.
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.”
