On the physiological side of things, it’s important to realize that the vast majority of your daily caloric burn comes down to just basic functions like breathing and keeping your heart beating, Moore says. Called your basal metabolic rate (BMR), your muscle does play a role in setting it, but extra muscle isn’t going to turn you into a supercharged calorie-torching machine. And even though exercise does burn calories, that total is often significantly less than what we expect and would need to create a large daily caloric deficit, he says.
The major reason HIIT works so well for dropping body fat is due to the greater calorie burn (or EPOC—excess post-exercise oxygen consumption) that's maintained after the workout is over. In other words, you burn more calories and more body fat while you're sitting around doing nothing. In addition to this increase in resting metabolism, HIIT is effective at enhancing the mechanisms in muscle cells that promote fat burning and blunt fat storage.
In a way, moderate-intensity physical activity is that "magic pill" a lot of people are looking for, because the health benefits go beyond keeping your waistline trim: Not only can it reduce your risk of cancer, stroke, diabetes and heart attacks, but studies have shown that physical activity can significantly improve the moods of patients with major depressive disorders.