Calories are units of energy that provide fuel for our bodies. The number of calories a person needs each day depends on factors like their metabolic rate, physical activity level, and the thermic effect of food. Recommended daily calorie allowances range from 1,600 calories for sedentary women and older adults to 2,000 calories for active teenage boys and men. To maintain weight, the number of calories consumed must equal the number burned through physical activity and basic bodily functions, with 3,500 excess calories equaling about 1 pound of fat gain.