Breaking a sweat on an empty stomach has long been touted as an effective approach toward fat loss. You may have even endured a hungry early-morning treadmill session in the name of a better burn.
The laymen’s theory is that when you exercise without food in your system your body has to burn fat for fuel. Easy peasy, right?
Not exactly. According to the Journal of the International Society of Sports Nutrition, fasted cardio may just be a hunger-inducing waste of time.
The study had 20 healthy, college-age volunteers perform one hour of steady-state, moderate-intensity exercise (they jogged on the treadmill) three mornings per week for four weeks—that’s 12 workouts total. All participants followed a specific diet plan throughout the four weeks, but 10 of the volunteers received a meal-replacement shake right before their workout while the other 10 worked out without having eaten since the night before.
There was no significant difference between the weight lost by the fasted cardio folks and those that drank a shake pre-workout, indicating that there’s no significant benefit to doing cardio when your tank is on empty, says study author Brad Schoenfeld, Ph.D., C.S.C.S.*D, Director of the Human Performance Lab at CUNY Lehman College.
The misunderstanding around fasted cardio started decades ago, when there were just a few short-term studies—researchers tested subjects for just a workout or two—that suggested fasted cardio burned more fat, according to Schoenfeld.
The logic made sense: When you perform cardio exercise after hours without food (in the early morning, for example) your body relies on fat to fuel you. So over time, you’ll lose more fat—right? Thing is, when you do eventually eat again, your body uses that food to replace the stored fuel it just lost, according to Schoenfeld. Which is why when researchers broadened the period of time they studied their subjects it became clear that fasted cardio wasn’t as effective as they thought. “Fat loss takes place over days, weeks, and months, not just in the minutes on the treadmill,” says Schoenfeld.
That’s why the best strategy for fat loss is to eat fewer calories than you expend (maintaining what’s called a ‘caloric deficit’), he says. Schoenfeld suspects that it was the extended 1200- to 1300-calorie diet plan his study participants followed which led to their fat loss.
It may have no real benefit, but is it bad for you? Well, that depends on the type of cardio you’re doing. You might get through moderate steady-state cardio a-okay, because your body can steadily oxidize your body’s fat and use it for fuel. But if you’re doing high-intensity interval training (HIIT) or strenuous lifting, your body can’t rely on your fat stores because fat cannot be utilized quickly enough to produce sufficient fuel for your muscles, according to Schoenfeld. Instead, your body needs to rely on fast fuel sources, like carbs and sugars, to keep you going, Schoenfeld explains. So if you haven’t eaten in a while—like, say, since last night—you’ll likely see a negative effect on your performance.