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If you’ve ever woken up at 5 a.m. to squeeze in a morning ride and haven’t stopped on the way out the door to scarf down a banana or an energy bar, then you’ve done fasted cardio.
Maybe not fueling before a ride is a scheduling necessity, and you want head out before your favorite trails or bike paths get too crowded during this time of social distancing. Or maybe you just can’t stomach eating that early. But maybe you’ve heard the buzz linking fasted cardio to weight loss and performance benefits. So, should you be exercising on an empty stomach? Here’s what the experts have to say.
What Is Fasted Cardio?
Fasted cardio isn’t a hard concept to understand. “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 people may claim they’re doing fasted cardio by skipping lunch and heading straight into a post-work ride, but the scientific literature specifies that the body needs a 10- to 14-hour period of not eating to be truly fasted, adds Meghann Featherstun, R.D., board-certified specialist in sports dietetics based in Cleveland. So, for most people, fasted cardio would occur first thing in the morning.
What’s the point of exercising on empty? Well, once you’ve been in a fasted state for a certain amount of time, there’s little to no glycogen available for energy, and your body needs to find another source of fuel. Within a few minutes of starting a fasted workout—no matter how far you’re planning on going—the percentage of fat you would burn in that workout would be a little bit higher, says Antonucci.
The research is clear: If we exercise 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 Featherstun. People who ran on a treadmill in a fasted state burned 20 percent more fat than those who ate in one small study published in the British Journal of Nutrition in 2013. And people who consistently trained in a fasted state over the course of six weeks showed more endurance improvements than those who ate before working out, an 2010 study published in the Journal of Applied Physiology found.
The Drawbacks of Exercising on Empty
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 bonk!).
When it comes to weight loss, all that fat-burning potential 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.
BTW, 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 Featherstun. Working out in a fasted state yielded twice the amount of protein breakdown in muscles than in a non-fasted state, according to a 2011 study published in the Strength and Conditioning Journal.
And about those endurance benefits… People were actually able to perform aerobically for longer after eating than when they fasted, a meta-analysis published in the Scandinavian Journal of Medicine & Science in Sports in 2018 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 workouts, and with that our dependence on carbs increases. “If we don’t have carbs to pull from, the intensity of our workout is going to suffer,” says Featherstun.
Any time you do any high-intensity workout, you’re going to burn a 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 exercising off fat versus carbs, says Featherstun, and you’re likely to finish fasted high-intensity workouts feeling terrible or hit a wall sooner in longer efforts.
So, Should You Do Fasted Cardio?
If you’d rather use your preride 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 workouts when you’re in a fasted state,” says Featherstun. Cap those low-intensity rides at about 45 to 60 minutes, which is about how long it will take before your glycogen stores get low.
Otherwise, when it comes to really putting in the work, it’s better to exercise within one to three hours after eating breakfast. “When you’re properly fueled, it’s going to feel better mentally and physically,” says Antonucci.
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