Why Are Calories Important?

Calories are a measure of energy and are commonly used to describe the energy content of foods. Your body is able to break down food molecules and use the stored energy for many different functions, including movement, thought and growth. Calories also play a role in weight control, because eating more calories than your body needs can lead to weight gain.

Metabolism refers to the process by which your body converts calories from food into energy it can use. Your body needs energy -- even while you are at rest -- for breathing, circulating blood, repairing cells and all other bodily functions. Making sure you eat enough food can help ensure that your body has enough energy to develop and work properly. Your metabolism and how many calories your body needs each day is influenced by your age, gender and physical activity habits. You can estimate your daily calorie needs by using a daily food plan, such as the USDA's MyPyramid program.

Keeping the amount of calories you consume balanced with the amount of calories you burn through regular metabolism and physical activity will keep your weight stable. When you take in more calories than your body burns, your body stores these extra calories as fat and you gain weight. Losing weight requires creating a calorie deficit so that your body burns more calories than you take in. Create this calorie deficit by eating 500 to 1,000 fewer calories than your body needs each day and participating in 30 minutes or more of physical activity that helps burn calories. For example, just sitting for one hour burns 81 calories for a 150 pound person, while strolling at a modest rate burns 206 calories and jogging burns 675 calories per hour. It takes using up 3500 calories over and above what you eat to lose 1 pound.

A man aged 31 to 35 who's moderately active needs about 2,600 calories daily, while a woman of similar age and activity needs about 2,000. Your body gets its calories from macronutrients, which are the nutrients your body needs in large amounts,including carbohydrates, protein and fat. One g of carbohydrate provides your body with 4 calories, as does 1 g of protein ... but, 1 g of fat provides your body with 9 calories. Approximately 45 to 60 percent of your daily calories should come from carbohydrates; 10 to 35 percent should come from protein; and 20 to 35 percent should come from fat.

Meeting your calorie needs with a nutritious, well-balanced diet can help keep your body healthy. The Harvard School of Public Health recommends a diet that includes plenty of vegetables, fruits, whole-grain foods and healthy sources of protein and fat. It also recommends skipping sugar-sweetened drinks that add calories to your diet without adding beneficial nutrients. Talk with a registered dietitian who can help you develop an eating plan that includes essential nutrients while meeting your specific energy needs.