Myth: “Low intensity aerobic exercise burns more body fat than high intensity aerobic exercise.”

This theory suggests that once your heart rate rises out of the “target fat-burning zone,” you cease to burn fat and you burn mostly carbohydrates. Therefore, the theory goes, the best way to lose fat is staying at a slower pace and a lower intensity to burn body fat.

The theory is that during low intensity exercise, you burn a greater percentage of calories from fat stores, so people assume low intensity exercise burns more fat overall. If this were true, we could extend the theory to it’s logical conclusion and say that sleeping for twelve hours a day is the ultimate fat burner because when you’re sleeping you’re burning the greatest proportion of fat. The problem is, because sleeping is so “low in intensity,” it hardly burns any calories! If the intensity of an activity is too low, you don’t burn enough total calories to have any impact on fat loss.

Why Moderate to High Intensity is Better

At lower intensities, you burn a greater percentage of calories from fat than carbohydrates, and at higher intensities you burn a greater percentage of calories from carbohydrates. High intensity aerobic exercise can use as much as 65-70% of the body’s energy needs in the form of carbohydrate. The most important issue for fat loss is not the ratio of fat to carbohydrate burned, but the total number of calories burned and high intensity aerobic exercise burns the most calories!

What about the other 23 hours of the day? If you burn more carbs during the workout, your body will compensate over 24 hours and burn more fat later on in the day instead of carbs.

The lower the intensity, the lower the total number of calories burned and the higher the intensity, the greater the number of calories burned. High intensity cardio also raises your metabolic rate after the workout to a much greater degree than low intensity cardio. That’s why high intensity cardio is better, provided that you can maintain it for a long enough duration to burn an appreciable number of calories.