Dietary fiber

Dietary fiber consists of plant carbohydrates such as cellulose, hemicellulose, pectin and resistant starch, together with the related compound lignin, that pass through the stomach and small intestine without being digested. Humans lack the enzymes needed to split the bonds joining their sugar units, so fiber supplies few calories, on average about half as many per gram as digestible carbohydrate. It is found in whole grains, legumes, vegetables, fruits, nuts and seeds, but not in meat or dairy foods.

Fiber is often divided into two types. Soluble fiber, plentiful in oats, barley, beans and many fruits, dissolves in water to form a gel that slows digestion, softens rises in blood glucose and helps lower LDL cholesterol. Insoluble fiber, found in wheat bran and whole grains, adds bulk to stool and speeds its passage through the gut, which helps prevent constipation. In the large intestine, gut bacteria ferment much of the fiber, producing short-chain fatty acids that feed the cells lining the colon.

Diets high in fiber are linked to lower rates of heart disease, type 2 diabetes, obesity and colorectal cancer. Many people eat well below the roughly 25 to 35 grams a day that guidelines suggest, and raising intake suddenly can cause gas and bloating.