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Fiber

By

Shilpa N Bhupathiraju

, PhD, Harvard Medical School and Brigham and Women's Hospital;


Frank Hu

, MD, MPH, PhD, Harvard T.H. Chan School of Public Health

Reviewed/Revised Feb 2023
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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.

Fiber in the diet helps mainly by adding bulk to the contents of the intestines. This helps make stool easier to pass and minimizes constipation Constipation in Adults Constipation is difficult or infrequent bowel movements, hard stool, or a feeling that the rectum is not totally empty after a bowel movement (incomplete evacuation). (See also Constipation... read more . Fiber also may provide satiety (the feeling of fullness and relief of hunger). However, eating too much insoluble fiber can make you feel bloated and also 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.

Table

Comparing Soluble and Insoluble Fiber

Type of Fiber

Sources

Functions

Soluble

Apples (mainly in the flesh of the apple)

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

Insoluble

Apples (mainly in the skin of the apple)

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

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

NOTE: This is the Consumer Version. DOCTORS: VIEW PROFESSIONAL VERSION
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