Fiber

Main article: Dietary fiber
Fiber is a carbohydrate (or a polysaccharide) that is incompletely absorbed in humans and in some other animals. Like all carbohydrates, when it is metabolized it can produce four Calories (kilocalories) of energy per gram. But in most circumstances it accounts for less than that because of its limited absorption.

Dietary fiber consists mainly of cellulose, a large carbohydrate polymer that is indigestible because humans do not have the required enzymes. There are two subcategories: soluble and insoluble fiber. Whole grains, fruits (especially plums, prunes, and figs), and vegetables are rich in dietary fiber.

Fiber is important to digestive health and is thought to reduce the risk of colon cancer.[citation needed] It can help in alleviating both constipation and diarrhea. Fiber provides bulk to the intestinal contents, and insoluble fiber stimulates peristalsis: the rhythmic muscular contractions passing along the digestive tract. Some soluble fibers produce a solution of high viscosity: a gel, which slows the movement of food through the intestines. Fiber, especially from whole grains, may help lessen insulin spikes and reduce the risk of diabetes [type 2].