Many forms of fasting have become popular in recent years due to research supporting its potential benefits for metabolic health. Some people skip food and drink for a certain number of hours á la intermittent fasting, while others exercise on empty, also known as fasted cardio. And while working out without fueling first might provide some payoffs, it definitely depends on your goals, the type of exercise you do, and how your body reacts to working out without food.
Here, we break down exactly what fasted cardio is, the potential pros and cons, and who might benefit most (or least) from the practice.
What is fasted cardio?
Simply put, fasted cardio means you don’t have anything to eat before you engage in physical activity, says Jamie Kyei-Frimpong, MS, FNP-BC, nurse practitioner at Parsley Health. You typically work out first thing in the morning, so you fast overnight for at least eight hours. The cardio itself can range from low intensity to high intensity. Typically, people start out with short bouts of fasted cardio, like 20 to 30 minutes, but others go for even longer.
What are the benefits of fasted cardio?
One of the biggest potential benefits of fasted cardio is that it can lead to fat metabolism or helping your body use fat as fuel instead of glucose, says Kyei-Frimpong. Theoretically, this may help you lose fat faster.
Research hasn’t quite caught up to this trend yet, though. While there have been studies on fasted cardio, the majority of that research involves less than 100 subjects, mostly men, and the results don’t consider long-term effects. However, one UK review paper stated that a single aerobic workout done in a fasted state can increase the utilization of fat as fuel, boost metabolism, and reduce energy intake throughout the day. (However, the researchers do note that more research is needed.)
Another study on just 12 men, found that skipping breakfast pre-workout lowered overall calorie intake for the day—but different research found that fat oxidation three hours after exercise was actually higher in those who ate before and feelings of hunger were also lower after a workout in those who ate before.
So while exercising without eating beforehand may provide some benefits, like potentially helping you cut back on fat and calories, research can’t say 100% that will happen for you or even most people.
Are there any downsides to exercising while fasting?
When you don’t have something to eat before you break a sweat, your body might rely on fat to power you through, but it could also turn to protein instead, Kyei-Frimpong explains. “That could mean you also lose muscle mass,” she says.
Also, if you’re training for a race or any sort of athletic event, it could negatively impact your performance because you’ll likely be low energy—especially if your body isn’t used to working out in a fasted state, when fuel isn’t readably available or easy to access. Your body’s preferred fuel source is glucose, so when it doesn’t have that, it takes a while to adjust to turning to fat instead. “It takes more energy to switch over to burn fat as fuel,” Kyei-Frimpong says. “If you think about runners in longer races, they have pure sugar and carbs, so their bodies burn that fast. If they’re handed an avocado, they don’t burn it as efficiently.”
One study, involving 20 male cyclists, found that when subjects worked out without eating first, it compromised their exercise intensity and volume. (Though interestingly, fasting increased their time to exhaustion.)
If you’re looking to do a high-intensity interval workout (or HIIT), Kyei-Frimpong suggests not doing it in a fasted state. She says she prefers people only do steady-state exercise while fasting. “When you’re working out at a high intensity, you need glucose for fuel in order to maintain that intensity. Otherwise, you could feel dizzy or weaker,” she says.
Keep in mind, research goes back and forth on the benefits (and drawbacks) of fasted exercise, so it really comes down to what works for you.
How to know if fasted cardio is right for you
If you decide you do want to try fasted cardio, Kyei-Frimpong suggests starting slow and short, going out for about 20 to 30 minutes for a run, bike, or elliptical workout. “Try it a few times a week, just to see how you feel, and then go from there,” she says. “If you’re not feeling well, it’s probably not the thing for you. If you are feeling well, try adding more time. But ideally, it’s not your main form of exercise, because you want to add intensity.”
It’s also important to know that it might take some time to see results from fasted cardio. You could do it one to two times per week—as long as you’re consistent, that’s when you’ll see the most benefit, Kyei-Frimpong says.
For those who prefer lifting weights to jogging or biking, Kyei-Frimpong says you definitely want to eat before a strength training session. “The energy and performance of lifting weights, especially when the goal is to lift on the heavier side to build muscle and have a higher ability to burn fat [at rest, which happens when your body has more muscle mass], then you need fuel in order to lift weights,” she says.
Also, anyone who has diabetes should stay away from fasted cardio, Kyei-Frimpong explains. Those with insulin sensitivities or abnormal glucose levels should eat on a regular basis, as should anyone who feels weakness or dizziness. Regular energy intake will help keep glucose in check and can help you avoid that feeling weak or off-balance feeling.
Most importantly, consider your goals when doing fasted cardio: “If your goal is fat loss, then OK, add in fasted cardio,” Kyei-Frimpong says. “If your goal is to do well at an event, then eat before.” Your physical performance will likely have an edge if you have some fuel in the tank.