Energy From Food Fuels Activity

Food and beverages provide the body with energy in the form of calories. The body uses those calories to fuel body functions and physical activity. People can control how many calories they consume in what they eat and drink, as well as how many calories they burn for physical activity. Because calories are used to measure energy, caloric balance is also called energy balance.

The body uses calories in three ways:

• Metabolism. The body uses calories to provide energy for growth, development, and maintenance of body tissues. Calories also fuel organ functions, such as heartbeat, breathing, growth, tissue repair, blood production, brain activity, and heat production. Some 60 percent to 75 percent of the calories converted by the body every day are used for metabolism, and we cannot influence the number of calories spent on these basic functions—with two exceptions. First, metabolic rate is influenced by the amount of one’s muscle tissue. Muscle burns more calories, even at rest, than fat tissue does; as a result, the more muscle one has, the more calories one spends on metabolism. Thus, a muscular person has a slightly higher, or faster, metabolism than does a person with a higher percentage of body fat. Second, metabolism increases for an hour or two after a period of physical activity; therefore, frequent activity helps boost one’s metabolic rate.
• Food digestion. About 10 percent of daily calorie usage fuels the process of breaking down food, digesting it, absorbing it, and transporting the nutrients to where they are needed in the body. The number of calories spent on digestion cannot be changed.
• Physical activity. Whether it’s walking to school, playing at recess, or doing chores around the house or yard, physical activity uses 15 percent to 30 percent of calories consumed. People can alter how many calories they use in physical activity—the more activity, and the longer and more intense it is, the more calories are burned.

Anytime a person eats more calories than are used in a day, the body stores the extra calories as fat. It doesn’t matter which foods have contributed the extra calories. However, fat in food contains more than twice as many calories per gram as do carbohydrate and protein. Thus, when fat is added to food, the food becomes much higher in calories, or more calorie dense. Fatty foods often taste good, which makes it easy to eat too much of them, in which case people end up eating more calories than are necessary to meet the body’s needs. The point to emphasize with students is that regardless of which foods one eats, if one consumes more calories than one burns, then the body stores the extra calories as fat.