Answer: The “fat-burning zone” lies between 50 and 70 percent of your maximum heart rate. When you exercise at this low intensity, your body draws energy from fat. As your heart rate goes up, more energy comes from carbs. So it seems logical that to lose fat you should keep your heart rate low. But that’s not the case.
Doing cardio at higher intensities causes you to burn a lower percentages of fat calories in favor of carbs, but you use more total calories. And that’s the key to slimming down. Plus, since you torch more total calories, the absolute amount of fat burned actually increases too.