Once you build muscle, if you don't continue to build it or keep it built, even a week off turns it all into fat. And if you managed to make those muscles bigger, not just more toned, when they turn into fat you get lots of fat. At least that's been my experience. So I don't think intensity is any kind of magic bullet. Even the sprinting seems not to have a lot of effect for body composition. It keeps me strong and gives me energy on the days I do it, but it seems not to have much of an effect otherwise.
Female, 5'3", 49, Starting weight: 163lbs. Current weight: 135 (more or less).
Starting squat: 45lbs. Current squat: 157.5 x 2