When you’re trying to lose weight, your gym time is all about the burn. But the kind of energy you're using to fuel your fire isn't all the same. Actually, there's a big difference between blasting fat and chiseling away kilojoules. Did we just blow your mind?
Fat and kJs aren’t the same thing. Kilojoules are a unit of energy, which can come from carbs, protein, or (you guessed it) fat, says dietitian and trainer Dr Albert Matheny.
What This Means for Your Workouts
Balls-to-the-wall, high-intensity workouts primarily burn kJs from carbs stored in your muscles, called glycogen. Low-intensity workouts, where you can still carry on a conversation, burn mostly kJs from fat, says Matheny.
Your body also burns a greater chunk of fat kJs when you work out in a fasted state, like first thing in the morning before breakfast. (Your body rarely burns calories from protein, which is good since protein is muscle. You don’t want to burn that!)
If your goal is to burn fat, it makes sense to work out at a low intensity, on an empty stomach, right? Not so fast. While working out at a lower intensity or in a fasted state will allow you to burn a greater proportion of fat, sweating it out at a higher intensity means burning more overall, he says.
For example, in one University of Wisconsin study, people who performed a 20-minute HIIT (high-intensity interval training) workout burned twice as many kJs as they did during long, low-intensity runs.
“If you want to lose weight, it’s the number of kilojoules you burn overall that matters the most,” says Matheny. “It doesn’t matter if they are from carbs or fat.” That’s because after you burn a ton of kJs from stored carbs during intense exercise, your body goes into overdrive to replace those carbs and repair your muscles. How does it do that? By breaking down fat and burning even more, he says.
That explains why, in one Journal of Sports Science and Medicine study, runners who performed high-intensity sprints lost more belly fat than those who worked out for the same amount of time each week, but stuck to low-intensity running.
Set Yourself Up for Fat Burn
And don’t even think about trying to churn out high-intensity workouts on an empty stomach to burn both fat and calories, says Matheny. In a fasted state, you will feel like you are working freakishly hard, but the actual amount of you burn will be super-low. Carbs are your body’s fastest form of energy. By fueling up before your workouts, you have more energy. “You're better able to achieve weight loss,” he says.
Give Low-Intensity Workouts Love
As great as high-intensity work is, you still need some low-intensity workouts to give your body a break, says Matheny. “Performing high-intensity training all of the time keeps your levels of the stress hormone cortisol elevated, leading to fat retention and muscle breakdown.” Plus, it ups your risk of injury. Not good for helping you achieve weight-loss wins.
For that reason, Matheny recommends performing about two high-intensity workouts per week on non-consecutive days. Dial down the intensity to the point that you can carry on a conversation during your other workout days. Hit up a Vinyasa yoga class, or use lighter weights and longer rest periods during a circuit weight-lifting workout. As you get in better shape, you can start upping your high-intensity training to three or four times per week.
But, whatever your workout schedule, mixing in some low-intensity routines will help your body recover properly. And when it comes to healthy weight loss, that’s what it’s all about.