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Considering Fasted Cardio? Here’s Everything You Need to Know.
It’s making headlines as the new way to burn fat and crush calories, but is working out on an empty stomach really a good strategy?
From two-a-day workouts to carb-loading race weeks, there is no shortage of strategies (some legit, some myths) for how to use food and exercise to maximize your fitness gains. But deciphering which ones are worth your time is another story. The newest strategy trending among pro athletes and celebs is known as fasted cardio. The technique, which has been used by everyone from bodybuilders to distance runners as a means of torching fat, has 535k tagged posts on Instagram. So what is it, exactly, and how does it work?
Understanding Fasted Cardio
Like it sounds, fasted cardio means you’re engaging in aerobic-based activity on an empty stomach. This isn’t an hour or two sans snack. For your body to be in a fasted state, you need to go without food for an 8- to 12-hour window, says Megan Ostler, R.D., a dietitian in Logan, UT. Basically, those mornings when you’ve laced up your sneakers for a run or hit an indoor cycling class before you’ve had a chance to scarf down breakfast, you’ve tested out this technique.
As for why you’d want to be under-fueled for your morning workout, the belief among some sports nutritionists is that fasted cardio is an effective fat-burning strategy. In a fasted state, your body’s glycogen stores—the go-to source for quick energy for your muscles—are drained. Normally, breakfast would replenish them, but if you don’t eat, it forces your body to look for alternative fuel sources. “Your body will naturally tap into other energy stores, like stored fat, in order to satisfy its energy needs,” explains certified fitness and nutrition coach Lauren Powers, co-owner of Total Fitness Revolution in Mableton, GA.
That means you’ll be burning more fat during your workout, as well as more calories, since your body has to work harder on a cellular level to tap into these alternative fuel sources.
What the Science Says
It sounds logical, and there is some science to back it up. A small study found that men who hit the treadmill after fasting overnight burned 20 percent more fat during their workout than their breakfast-eating counterparts. But other data, including a 2017 meta-analysis in the Journal of Functional Morphology and Kinesiology, found that the fat-loss and calorie-burn difference between fasted and fueled training is minimal. And other research in the Journal of the International Society of Sports Nutrition found that women who followed a regular cardio workout plan lost roughly the same amount weight and body fat regardless of whether or not they ate beforehand.
Then there’s the whole issue of calories in, calories out. “Ultimately, weight loss is about creating a calorie deficit throughout the day,” says trainer Frank Baptiste, owner of Frankly Fitness in New York City. “What happens in 20 minutes or an hour-long workout matters much less than what happens over the course of a day or days. It’s all about energy balance—calories in versus calories out—regardless of what percentage comes from fat.”
In other words, whether you eat before or after your workout is likely less important if you’re trying to lose weight than simply eating fewer calories overall.
Fasted Cardio and Performance
Skipping your pre-workout fuel could be a questionable call for performance as well. Research shows exercising on empty negatively affects both your ability to go hard and go long. And it comes with risks, including dizziness, lightheadedness and fatigue. Women need to be careful with this approach as it can affect hormones, says Ostler (and you should definitely avoid it if you’re pregnant).
The truth is, whether you’re a weekend warrior or CrossFit champion, fueling is paramount to perform at your peak during a competition, says Ryan Turner, R.D., Director of Sports Nutrition at Tone House in New York City and founder of Food is Fuel NYC. “The special needs of an athlete include higher carbohydrates, higher protein, and higher overall calories due to training,” he says. When activity exceeds 45 minutes without fuel, says Turner, not only will performance suffer but you may also begin to compromise muscle mass.
And here’s something else to consider: Lack of fuel can affect not just your performance, but how you feel about it. Research shows that individuals who fast before a cardio session perceive their level of exertion to be significantly higher than those who are properly fueled. What that means is that even though two people are doing the same workout or racing the same event, the one who fasted beforehand will feel as though their effort is a lot harder for the same result. “This can have a cumulative effect on the mind, making some people doubtful of their abilities and deter them from wanting to continue,” says Turner.
If, after weighing the pros and cons, you decide fasted cardio is still a trend you want to try, you need to pay special attention to what you eat immediately following your workout. Refueling, repairing and rehydrating are key, says Turner. “You’ll want to consume carbohydrates and protein at a two-to-one or at least one-to-one ratio following your workout to replenish stored energy in the muscles,” he says. “Also, make sure you consume adequate protein—about 0.2 grams per pound is a solid rule—to maintain and repair muscle that has been stressed during activity.”
For a 140-pound woman, that’s 28 grams of protein, the equivalent to 1½ cups of Greek yogurt, or a three-egg omelet with low-fat cheese. Also, make sure you replace fluids you sweated out by drinking 16 ounces for every pound lost during activity, says Turner.
Ultimately “there’s no cookie cutter approach to how you diet and train for performance,” says Power. So whether you find yourself forgoing food or getting your grub on, the important thing is to listen to your body and act accordingly.