If you imagine that food is like wood and your metabolism is like fire, then calories are the heat that burning the wood produces. For example, a woman who weighs 130 pounds would require 1,300 calories per day just to maintain her bodily functions. That percentage varies widely depending on a person's activity level. Moderately fit and active people might need 30-50 percent calories above their BMR to maintain their current weight. Those who are very fit and exercise frequently might burn as much as 100-200 percent more than their BMR, which means they have to eat about two to four times as much as the average person just to maintain their weight.
Exercise is absolutely essential to keep body fat down.
Simply being thin doesn't guarantee that you are healthy and in good shape. Both play important roles in shedding pounds and maintaining a desired weight. Aerobic exercise spurs your body to burn fat efficiently for energy. That is because oxygen is an essential ingredient in metabolism. Jogging, swimming, biking, aerobic dancing and cross-country skiing are all forms of aerobic exercise.