What Is Fiber?

Fiber, or "bulk" or "roughage," is a substance found in plants. Dietary fiber -- the kind you eat -- is found in fruits, vegetables, and grains. It is an important part of a healthy diet. Dietary fiber adds bulk to your diet. Because it makes you feel full faster, it can be helpful in controlling weight. Additionally, fiber aids digestion, helps prevent constipation, and is sometimes used for the  treatment of diverticulosis, diabetes, and heart disease.

There are 2 forms of fiber: soluble and insoluble.

Soluble fiber attracts water and turns to gel during digestion. This slows digestion. Soluble fiber is found in oat bran, barley, nuts, seeds, beans, lentils, peas, and some fruits and vegetables. Soluble fiber has been scientifically proven to lower cholesterol, which can help prevent heart disease.

Insoluble fiber is found in foods such as wheat bran, vegetables, and whole grains. It appears to speed the passage of foods through the stomach and intestines and adds bulk to the stool.

(Information on this page was adapted from the MedlinePlus Medical Encyclopedia, a service of the U.S. National Library of Medicine and the National Institutes of Health.)

How Does Fiber Do What It Does?

Insoluble fiber binds water, making stools softer and bulkier. Therefore, fiber, especially that found in whole grain products, is helpful in the treatment and prevention of constipation, hemorrhoids and diverticulosis. Diverticula are pouches of the intestinal wall that can become inflamed and painful. In the past, a low-fiber diet was prescribed for this condition. It is now known that a high-fiber diet gives better results once the inflammation has subsided.

Low blood cholesterol levels (below 200 mg/dl.) have been associated with a reduced risk of coronary heart disease. The body eliminates cholesterol through the excretion of bile acids. Water-soluble fiber binds bile acids, suggesting that a high-fiber diet may result in an increased excretion of cholesterol. Some types of fiber, however, appear to have a greater effect than others. The fiber found in rolled oats is more effective in lowering blood cholesterol levels than the fiber found in wheat. Pectin has a similar effect in that it, too, can lower the amount of cholesterol in the blood.

Other claims for fiber are less well founded. Dietary fiber may help reduce the risk of some cancers, especially colon cancer. This idea is based on information that insoluble fiber increases the rate at which wastes are removed from the body. This means the body may have less exposure to toxic substances produced during digestion. A diet high in animal fat and protein also may play a role in the development of colon cancer.

High-fiber diets may be useful for people who wish to lose weight. Fiber itself has no calories, yet provides a "full" feeling because of its water-absorbing ability. For example, an apple is more filling than a half cup of apple juice that contains about the same calories. Foods high in fiber often require more chewing, so a person is unable to eat a large number of calories in a short amount of time.

(Information on this page was adapted from Nutrition Resources Online Fact Sheet No. 9.333, Colorado State University Extension.)

How Much Fiber Is Enough?

The average American now eats 10 to 15 grams of fiber per day. The recommendation for older children, adolescents, and adults is 20 to 35 grams per day. Younger children will not be able to eat enough calories to achieve this, but introducing whole grains, fresh fruits, and other high fiber foods is suggested.

To ensure adequate fiber intake, eat a variety of foods, including more fruits, vegetables, whole grains, cereals, and dried beans and peas. Add fiber gradually over a period of a few weeks to avoid abdominal discomfort. Water aids the passage of fiber through the digestive system. Drink plenty of fluids (approximately 8 glasses of water or noncaloric fluid a day).

Peeling can reduce the amount of fiber in fruits and vegetables. Cooking may actually increase your fiber intake by decreasing the volume of the food that is prepared. Eating fiber-containing food is beneficial whether it is cooked or raw.

(Information on this page was adapted from the MedlinePlus Medical Encyclopedia, a service of the U.S. National Library of Medicine and the National Institutes of Health.)