The Center for Weight Management

Metabolism is the amount of energy, or calories, your body uses daily.
The majority of calories spent in a day (60-70 percent) go to your resting metabolic rate (RMR), which keeps your organs working, your heart pumping, and your temperature regulated.
Another 10 percent of calories are spent in your digestive system, allowing your body to break down the foods you consume.
The remaining 20-30 percent of calories are left for you to spend through physical activity.
Overweight people do not necessarily have slow metabolisms. Rather, their metabolic rates are usually faster. This is because their bodies have to work harder to keep them functioning regularly.
Building muscle mass can increase your RMR—but not by much. Your muscles only control a very small part of your RMR.
Some supplements can increase your metabolism, but as with muscle mass, they do not make a significant difference.
Burning away extra calories through increased physical activity is the best way to go about losing weight.
When it comes to dieting, as you cut back on your calorie intake your RMR will slow in an effort to conserve energy. This means your body is preparing for starvation. During this time physical activity paired with moderate strength training can maintain your RMR and aid in your weight loss.

Sources: Miriam Nelson, PhD, director of the Center for Physical Activity and Nutrition at Tufts University, Pamela Peeke, MD, assistant clinical professor of medicine at the University of Maryland, andClaude Bouchard, PhD, obesity researcher and Pennington’s executive director.