Metabolism: Converting food into energy

Metabolism is the process by which your body converts what you eat and drink into energy. During this complex process, calories in food and beverages are combined with oxygen to release the energy your body needs to function.

Even when you're at rest, your body needs energy for all its "hidden" functions, such as breathing, circulating blood, adjusting hormone levels, and growing and repairing cells. The number of calories your body uses to carry out these basic functions is known as your basal metabolic rate — what you might call metabolism.

Several factors determine your individual basal metabolism, including:

• Your body size and composition. People who are larger or have more muscle burn more calories, even at rest.
• Your sex. Men usually have less body fat and more muscle than do women of the same age and weight, which means men burn more calories.
• Your age. As you get older, the amount of muscle tends to decrease and fat accounts for more of your weight, slowing down calorie burning.

Energy needs for your body's basic functions stay fairly consistent and aren't easily changed.

In addition to your basal metabolic rate, two other factors determine how many calories your body burns each day:

• Food processing (thermogenesis). Digesting, absorbing, transporting and storing the food you consume also takes calories. About 10 percent of the calories from the carbohydrates and protein you eat are used during the digestion and absorption of the food and nutrients.
• Physical activity. Physical activity and exercise — such as playing tennis, walking to the store, chasing after the dog and any other movement — account for the rest of the calories your body burns up each day. Physical activity is by far the most variable of the factors that determine how many calories you burn each day.

Scientists call the activity you do all day that isn't deliberate exercise nonexercise activity thermogenesis (NEAT). This activity includes walking from room to room, activities such as gardening and even fidgeting. NEAT accounts for about 100 to 800 calories used daily.