Building muscle mass usually requires some sort of weight-bearing or resistance work such as lifting weights, using exercise bands or hand weights, and other similar forms of exercise. The amount of weight-bearing exercise you do has an effect on your metabolism. By the time 2 pounds of muscle is gained - usually over 3 months or so - your extra muscle can burn as much as an extra 65 calories a day. One study actually found that lifting weights boosted the resting metabolic rate by 9% over 18 weeks by adding 4 pounds of muscle mass. Ric Rooney and Bart Hanks of the Physique Transformation Web site say that a pound of muscle costs up to 50 calories a day to maintain, and a pound of fat costs just 2. So the greater the proportion of muscle to fat, the more efficient your metabolism is at burning fat. Muscle cells are as much as eight times more metabolically active than fat cells.

The efficiency of your metabolism is affected by a number of factors. Bottom line: overweight people just don't burn as many calories as people of normal weight. In most overweight people, either basal metabolism is lower, activity is less, and/or the thermic effect of food is blunted. So it's clear that the problem for many must be on the output side of the equation. Many overweight people do not eat any more than people of average weight. For example, if you are eating 2,000 calories a day, 10% of 2,000 is 200, so optimally you would be burning 200 calories a day simply eating and digesting your food. Thermic effect of food - about 10% of calories are spent processing the food you eat.