Fallacy: The amount of bodyfat you carry affects your metabolism.
Fact: The amount of muscle you carry in your overall body composition determines your metabolic rate.
In general, when someone refers to her metabolism, she's talking about her resting metabolic rate (RMR)-the amount of calories needed to sustain all the body's operations (maintain temperature, transport nutrients in and out of cells, pump blood, breathe, etc.) at rest. And the strongest predictor of metabolism is our fat-free mass, says David C Nieman, PhD, FACSM, director of the Human Performance Laboratory at Appalachian State University (Boone, North Carolina). "Fat-free mass is everything but the fat tissue," he explains. "It's predominantly made up of muscle but also includes bone tissue and water contained in the body."
It's the muscle that makes all the difference. For instance, if you were to compare your metabolic rate to that of a sedentary woman weighing the same, you'll likely burn more calories at rest than she will because you have more muscle and she probably has more fat due to her inactivity.
"Muscle tends to be very metabolic, in terms of burning calories, compared to fat; fat is not an inert tissue, but it doesn't expend nearly the amount of calories as muscle," says Robert Keith, PhD, RD. FACSM, professor of nutrition and food science at Auburn University (Auburn, Alabama). "When you think about fat's job, it's actually to store energy. It isn't going to be a tissue that burns a lot of calories because that would be counterproductive. So metabolism is very much tied up into body composition, and the more muscle you have, the more likely you are to have a higher resting metabolism."
Metabolism 101, part 2
Metabolism 101, part 3
Metabolism 101, part 4