Carbohydrates are probably the most important of the three main classes of foods since they are our main source of energy and should constitute at least 50–60 percent of the diet. There has been a shift in this century away from the healthful consumption of fresh fruits and vegetables and complex carbohydrates—the starches and fiber foods—toward a diet of more refined carbohydrates and simple sugars that are implicated in a variety of diseases, among them obesity, diabetes, cardiovascular problems, and tooth decay.

Carbohydrates are organic molecules; that is, they contain carbon and come from living sources. They are composed of carbon (C), hydrogen (H), and oxygen (O) (thus, the abbreviation CHO) in a 1:2:1 ratio. The basic relationship is that of carbon coupled with water molecules. Carbohydrates are a quick source of energy for the body, being easily converted to glucose, the fuel for the body’s cells. Each gram of carbohydrate releases four calories, units of heat or energy, for the body.

Carbohydrates are produced by photosynthesis in plants. The carbohydrates are the primary source of energy in nature’s plant foods—fruits, vegetables, grains, legumes, and tubers. These foods play a very important role in the functioning of the internal organs, the nervous system, and the muscles. They are the best source of energy for endurance athletics because they provide both an immediate and a time-released energy source as they are digested easily and then consistently metabolized in the bloodstream.

Carbohydrates are also needed to regulate protein and fat metabolism. With the proteins and fats, the carbohydrates help to fight infections, promote growth of body tissues such as bones and skin, and lubricate the joints. Many carbohydrate foods are also high in fiber, and the fiber content of foods is important in the bulking of the stool, which aids in regular elimination of waste materials through the colon. Indeed, fiber is thought to be helpful in preventing colon diseases such as colon cancer and diverticulosis and is being prescribed by more doctors as a dietary necessity.

Three principal carbohydrates are present in foods. Carbohydrates are classified according to their structure. First are the sugars, both monosaccharides (simple sugars), such as those found in honey and fruits, and oligosaccharides (multiple sugars), such as table sugar and malt sugar, which both happen to be disaccharides (two-sugar molecules). Then there are the starches, or complex carbohydrates, found primarily in vegetables such as carrots and potatoes and in whole grains such as rice and corn. Finally, there is fiber, mainly cellulose and hemicellulose, the indigestible roughage found in most unprocessed, carbohydrate-containing foods.

Sugars
The basic unit of the simple sugars (monosaccharides) is one hexose (containing six carbon atoms) or pentose (five carbon atoms) molecule. These simple sugars are easily and quickly digested and utilized by the body. They have the same chemical makeup but vary in structure. The disaccharides, such as table sugar or milk sugar, require some enzymatic breakdown but are easily converted into monosaccharides for digestion. The following represent the common basic sugars.