Muscle uses about 3 times as many calories per day as fat!
The basic principle here is that, since lean mass (muscle and bone) requires more energy to maintain than fat, resting energy expenditure is going to be higher with greater lean mass. If you take two men of equal weight, but one has a higher ratio of lean body mass to fat mass, he will burn more calories at rest than his chubbier counterpart. In terms of numbers, this works out to about 13 kcals/kg per day for muscle and 5 kcals/kg per day for fat. It is accepted that 80 percent of the difference in energy expenditure is explained by body composition. The remainder is most likely determined by genetics, age and gender.
To keep focused on the question at hand, let’s look at two studies that address the main ways people try to change their body composition: diet and exercise.
A study at Tufts University in 1994 found that men and women who participated in a resistance-training (strength-building) program gained lean mass and subsequently burned more calories at rest, regardless of how many calories they had burned during exercise. Interestingly, however, in a 1997 study at Columbia University, subjects were placed on a diet and randomly assigned to either a resistance-training or aerobic program and tracked throughout. The results found no significant differences in resting energy expenditure between the two groups. Why did one study show a metabolic boost with increased lean-body mass, while the other didn’t? One possible explanation is that when caloric intake is reduced as is the case with diets the resulting slowdown of the body’s metabolism overrides the boost that lean mass might provide. It is important, of course, to keep in mind that these studies had different experimental conditions.
My take: Resistance training increases lean mass, which results in more calories burned throughout the day; aerobic exercise burns more calories during a workout than resistance training; and the benefits of both are too important to ignore.