Get full access to Outside Learn, our online education hub featuring in-depth fitness and nutrition courses and more than 2,000 instructional videos when you sign up for Outside+.
Ask any collection of gym rats milling around the squat rack about the best way to burn fat and you’ll find two camps, those who swear by fasted cardio — done on an empty stomach first thing in the morning — and those who don’t. So what’s the deal? Does fasted cardio work, and if so, what exactly does it do?
The rationale for fasted cardio makes logical sense: When you fast (as you do when sleeping), your circulating blood sugar and glycogen stores fall. This drop in blood sugar is accompanied by a decrease in insulin levels, which should theoretically help the fat-burning process since insulin suppresses fat metabolism. Therefore, doing cardio on an empty stomach means your body needs to turn to another source of fuel. Fat seems a likely choice because it’s dense, it’s plentiful and (grr) it’s always there. But unfortunately, muscle tissue is more readily broken down than fat in the absence of glycogen, which means fasted cardio could potentially be a catabolic activity. In fact, the Strength and Conditioning Journal cited that protein breakdown could actually double during fasted cardio. Yikes.
Logic notwithstanding, the hard research on this subject is equivocal. On the pro side are studies like one published in the British Journal of Nutrition that showed participants who did fasted cardio burned 20 percent more fat than when they did cardio with a meal in their bellies. On the con side are studies like one from the International Journal of Sport Nutrition and Exercise Metabolism in which fed participants showed an increase in oxygen consumption (VO2) and substrate utilization (fat usage) both during the workout and in the 12 to 24 hours postworkout versus participants who fasted.
Confused yet? The hard truth is that fat loss isn’t as simple as a light switch that you can flip on and off depending on the kind of exercise you do. There are many other factors — metabolic, dietary and lifestyle — that come into play. A better way of looking at cardio and fat loss may not be within the workout itself but in your training week as a whole.
Fat loss really comes down to calories in versus calories out and, of course, a delicate balance of conscious nutritional decisions that fuel your body, build your muscle and stoke your metabolism. Choosing activities with a high metabolic deficit, like HIIT, produces an insane afterburn that lasts up to 48 to 72 hours postworkout and burns twice the fat of a steady-state workout in half the time. Do two to four HIIT workouts per week in conjunction with a solid strength-training program and you’re on the road out of Fat City.
So why not do HIIT in a fasted state to burn even more fat? It sounds like a good idea, and your body will break down lots of fat during a HIIT session, but the rate of that breakdown will usually exceed your ability to utilize those free-ranging fatty acids, and if they’re not used up they simply get repackaged into your fat cells. As for the afterburn, remember: For HIIT to be effective it should be done with all-out, maximal intensity. Training on an empty stomach often means a much reduced intensity level, however, so your HIIT might not be very burn-worthy and therefore less effective.
Where does that leave us? With a matter of preference. Some people prefer to work out on an empty stomach while others feel like they’re going to bonk if they don’t have a little breakfast. And while fasted cardio is not inherently bad for you, it probably isn’t your best option when it comes to losing body fat.