Carbohydrates are compounds that consist of carbon, hydrogen, and oxygen. These elements are linked together by energy-containing bonds. There are two types of carbohydrates: complex and simple. The complex carbohydrates, such as starch and fiber, are classified as polysaccharides. Simple carbohydrates are known as sugars and they are classified as monosaccharides or disaccharides, depending upon the number of sugars present. Monosaccharides consist of only one sugar, while disaccharides have two sugar molecules bonded together.
The purpose of the digestive system is to reduce all carbohydrates to the simple sugar glucose, which is readily absorbed into the bloodstream. Glucose, a monosaccharide, is the fuel of choice all the cells in the body rely upon. Complex carbohydrates take longer to digest; they must be broken down in stages. Most of the absorption of glucose produced from complex carbohydrates takes place in the large intestine. Simple sugars must be broken down into glucose as well; most of the glucose from simple carbohydrates is absorbed through the walls of the small intestine.
One of the complex carbohydrates used in the human diet is fiber. Fiber is a polysaccharide, in which the bonds holding it together can not be digested by humans. Fiber may be both water-soluble and water-insoluble. The main function of fiber is to bind bile acids, which reduces both fat and cholesterol aborption. Sources of fiber are wheat bran, whole grains, and brown rice.
Fiber is extremely important to the human diet. It aids weight control by displacing calorie-dense fats in the diet. Fiber also absorbs water and slows the movement of food throughout the digestive tract, giving a feeling of "fullness". Recommended intake of fiber should be somewhere in the range of 25 to 40 grams per day.