Merck Manual

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Adrienne Youdim

, MD, David Geffen School of Medicine at UCLA

Last full review/revision Aug 2019| Content last modified Aug 2019
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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.

  • 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.


Comparing Soluble and Insoluble Fiber

Type of Fiber




Apples (mainly in the flesh of the apple)



Citrus fruits


Oat bran


Pectin (from fruit)


Rice bran


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 atherosclerosis (including coronary artery disease)


Apples (mainly in the skin of the apple)

Brown rice



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

Makes people feel less hungry because it adds bulk to the diet, makes people chew more slowly, and keeps food in the stomach longer—and is thus helpful in losing weight

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