Some foods contain fiber, which is a tough complex carbohydrate. Fiber may be partly soluble: It dissolves in water, and the body may be able to digest some of it. Or it may be insoluble: It does not dissolve in water, and the body cannot digest it. Eating too much insoluble fiber can interfere with absorption of certain vitamins and minerals.

Authorities generally recommend that about 30 grams of fiber be consumed daily. In the United States, the average amount of fiber consumed daily is about 12 grams because people tend to eat products made with highly refined wheat flour and do not eat many fruits and vegetables. An average serving of fruit, a vegetable, or cereal contains 2 to 4 grams of fiber. Meat and dairy foods do not contain fiber.

Did You Know... Eating a lot of insoluble fiber (in such foods as brown rice, prunes, and many vegetables) can reduce the absorption of certain vitamins and minerals.

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Comparing Soluble and Insoluble Fiber Type of Fiber Sources Functions Soluble Apples Barley Beans Citrus fruits Lentils Oat bran Oatmeal Pectin (from fruit) Psyllium Rice bran Strawberries Helps moderate the changes in blood sugar and insulin levels that occur after eating a meal Helps reduce cholesterol levels May reduce the risk of coronary artery disease Insoluble Apples Brown rice Pears Prunes Many vegetables, including cabbage, root vegetables, and zucchini Whole grains and whole-grain breads and pastas Provides bulk to feces and thus helps food move through the digestive tract, preventing constipation Helps eliminate cancer-causing substances produced by the bacteria in the large intestine Reduces pressure in the intestine, helping prevent diverticular disease Is helpful in losing weight because the body processes it slowly

Last full review/revision July 2008 by Margaret-Mary G. Wilson, MD