Protein and fat calories also supply energy to the body. These two fuels have very specific jobs. Proteins help to maintain the body's immune system, form enzymes, hormones and other metabolic necessities. Fats are necessary to maintain the nervous system, cell membranes, hormones, absorb fat-soluble vitamins and aid in metabolism. Your calorie needs are determined by your body's metabolism, or metabolic rate.
Metabolism is roughly the number of calories the body needs to maintain itself each day.
This number is influenced by factors such as age, gender, muscle mass and activity level. A typical calorie need may be 2,000 per day. Let us assume that Joe's calorie input from food is close to his calorie needs, or metabolism. If this is so, Joe's body will have enough energy to maintain itself. These carbohydrates will provide energy to his brain and cells and supply enough excess carbohydrate to be stored in his liver and muscles as glycogen for later use between meals.