Metabolism: Converting food into energy

Metabolism is the process by which the body changes food and drink into energy. During this process, calories in food and drinks mix with oxygen to make the energy the body needs.

Even at rest, a body needs energy for all it does. This includes breathing, sending blood through the body, keeping hormone levels even, and growing and repairing cells. The number of calories a body at rest uses to do these things is known as the basal metabolic rate, also called basal metabolism.

Muscle mass is a key factor in basal metabolic rate. Basal metabolic rate also depends on other factors such as:

• Body size and composition. People who are larger or have more muscle burn more calories, even at rest.
• Sex. Men usually have less body fat and more muscle than women of the same age and weight. That means men burn more calories.
• Age. With aging, people tend to lose muscle. More of the body's weight is from fat, which slows calorie burning.

Besides the basal metabolic rate, two other things affect how many calories a body burns each day:

• How the body uses food. Digesting, absorbing, moving and storing food burn calories. About 10% of calories eaten are used for digesting food and taking in nutrients. You can't change this much.

• How much a body moves. Any movement, such as playing tennis, walking to a store or chasing the dog, makes up the rest of the calories a body burns each day. You can change this a lot, both by doing more exercise and by moving more during the day.

  Daily activity that isn't exercise is called nonexercise activity thermogenesis (NEAT). This includes activities such as walking around the house, gardening, housework and even fidgeting. NEAT accounts for about 100 to 800 calories used daily.