Most people, however, are a combination of all three, with a tendency to be more like one or two of them. Some people have a naturally high metabolic thermostat they tend to burn more calories than average even when they're asleep. Other individuals need fewer calories for the same physical activity. These metabolic differences alone, however, aren't great enough to account for weights that exceed the healthy ranges in weight tables. One explanation is that men have more muscle and less fat than women.
Muscle uses more energy than fat.
Because men have more muscle, they burn between 10 and 20 percent more calories than women during rest. As you get older, the amount of muscle tends to drop, and fat accounts for a greater percentage of your weight. Metabolism also slows naturally with age. Together, these changes reduce your calorie needs, often adding an extra pound a year after age 35. During the first few years of life, you form new fat cells rapidly.