Each of the macronutrients—carbohydrates, proteins, and fats—plays various roles in the function of our bodies. In addition to their unique functions, all of the macronutrients supply calories. When we eat more protein, carbohydrate, or fat than we need to replenish what we have used, the excess is converted to and stored as fat. Calories are used to support all muscular activity, to carry out the metabolic reactions that sustain the body, to maintain body temperature, and to support growth. But when we consistently take in more calories than we use, we gain weight. Weight is maintained when energy (calorie) intake balances energy output (see Chapter 3, page 48).
The carbohydrates are a vast and diverse group of nutrients found in most foods. This group includes simple sugars (like the sugar you add to your morning coffee) and complex forms such as starches (contained in pasta, bread, cereal, and in some fruits and vegetables), which are broken down during digestion to produce simple sugars. The main function of the simple sugars and starches in the foods we eat is to deliver calories for energy.
The simple sugar glucose is required to satisfy the energy needs of the brain, whereas our muscles use glucose for short-term bouts of activity. The liver and muscles also convert small amounts of the sugar and starch that we eat into a storage form called glycogen. After a long workout, muscle glycogen stores must be replenished.
Both simple sugars and starches provide about 4 calories per gram (a gram is about the weight of a paper clip). Because carbohydrates serve primarily as sources of calories (and we can get calories from other macronutrients), no specific requirement has been set for them (see Chapter 1, The Dietary Reference Intakes [DRIs], page 5).
But health experts agree that we should obtain most of our calories (about 60 percent) from carbohydrates. Our individual requirements depend on age, sex, size, and activity level.