When you’re in a moderate, low-intensity exercise state—an aerobic state—your body can keep up with the amount of activity it’s being asked to do. But with HIIT, you’re asking more and more and more from your body, and that comes with a lot of extra calories burned.2 It burns more fat in less time.
In addition to burning more calories, HIIT workouts also help you burn more fat in less time than endurance activities. That's because it causes your body to start burning fat for energy instead of carbs.3 Keeps burning cals, even after you're done working out.
When you do a "sustainable" exercise, like going on a long run, your body uses less oxygen than when you push yourself to the max, as you do during HIIT. And when you work out to the point where it's difficult to breathe (because your body is using more oxygen), your metabolism starts working at a higher level, which helps you burn more calories even after you stop exercising.4 Improves your heart.
When you turn up the intensity on your workout, as HIIT does, you're making your heart work harder, and since this muscle (like most) becomes stronger after putting it through tough training, HIIT is very beneficial for your heart health.5 Lose fat not muscle.
With interval training, you're pushing yourself as hard as you can go to accomplish as much work as possible. That butt-busting earns you cardiovascular benefits. By incorporating strength-training exercises into your intervals (as well as cardio moves), you also reap muscle-strengthening benefits.