Yes. But a slow metabolism is uncommon.

Metabolism is the complex biochemical process by which the food you eat is converted into the energy your body needs to function. Carbohydrates, proteins, fats and other nutrients combine with oxygen to release energy. Metabolism also produces heat, carbon dioxide, water and waste products. Your body uses the energy created for muscular activity and for many essential chemical processes. The heat it generates helps maintain body temperature.

Basal metabolic rate (BMR) is the amount of energy your body uses at rest. This rate is based on calories burned. The number of calories burned depends on how much you exercise and on your body's muscle-to-fat ratio as well as on your BMR. All physical activity, not just strenuous exercise, increases your calorie burn.

The more muscle and the less fat you have, the more calories you burn. That's one reason why experts recommend strength training as a way to build muscle mass. It's also why men — who typically have more lean muscle than women do — tend to lose weight faster and gain it more slowly. Metabolism slows with age. This is most likely due to a loss of muscle mass.

Many people who've had difficulty losing weight blame it on slow metabolism. In fact, only a small number of people burn fewer calories at rest than they should, according to estimates based on their height, weight and age. Because people who weigh more have more muscle as well as fat, overweight people usually have an increased metabolic rate, contrary to popular belief.

Some medical conditions can decrease BMR or alter the muscle-to-fat ratio. These include underactive thyroid (hypothyroidism) and Cushing's syndrome. Your doctor can test for these conditions.