While you and I say "fast metabolism," scientists use the phrase high "basal metabolic rate," or the amount of calories the body needs while resting. The misconception is that thin people always have a high basal metabolic rate.
It is true that muscle requires more calories than fat. In fact, a pound of fat burns about 10 calories and a pound of muscle burns nearly 30, said Catherine Kotz, who studies how the brain regulates energy balance at the Veterans Affairs Medical Center and the University of Minnesota in Minneapolis.
What matters, though, are the relative amounts of fat and muscle. Obese people with lots of fat can have a higher basal metabolic rate than lean individuals who have little muscle and fat.
Kotz explained with a very simplistic example: six pounds of fat would burn 60 calories, while one pound of muscle would only burn half that much.
However, if two people have the same amount of fat, the person with more muscle would have a higher "metabolism."