Metabolism, the process by which our bodies burn calories (food energy), has three parts: resting metabolic rate, the thermic effect of food, and physical activity. Resting metabolic rate (RMR) is the energy we use at rest to perform basic body functions like breathing and sleeping. In most people, this accounts for about 60 to 70 percent of their total daily energy expenditure (about 1,450 calories a day for a 140-pound woman). Because muscle is the body’s metabolically active tissue, RMR is almost totally determined by the amount of muscle or lean body mass. Most women have more body fat in proportion to muscle mass than men, and thus women generally have metabolic rates 5 to 10 percent lower than men of the same height and weight. Men therefore use more calories when they sit on a bench than the women sitting next to them.
The RMR of most people goes down by 2 to 3 percent with each decade after age thirty.
We can prevent this loss with regular strength-training exercises, designed to build or preserve muscle.
The thermic effect of food (TEF) is the energy we use to burn calories, to digest, absorb and metabolize our food. When you eat a 110-calorie snack, for example, 10 of those calories are used for TEF. It is a relatively small portion of our total metabolism: about 10 percent, or 240 daily calories, for a 140-pound woman.
Our greatest control over metabolism is to increase physical activity. In our sedentry culture of drive-through banks, escalators, leaf blowers, the TV and the computer it takes intention to move enough to burn calories. Exercise and a healthy diet made up of whole foods and lots of vegetables and fruits keep us well.