How hot are you burning? The number of calories your body uses to carry out these basic functions is known as your Basal Metabolic Rate — what one might call metabolism. Several factors determine your individual basal metabolic rate:
Your body size and composition. The bodies of 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, burning 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. Your basal metabolic rate accounts for about 60 to 75 percent of the calories you burn every day.
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. This accounts for about 10 percent of the calories used each day. For the most part, your body's energy requirement to process food stays relatively steady and isn't easily changed.
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
Metabolism is the process by which your body converts what you eat and drink into energy. During this complex biochemical 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.
