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Fiber 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. Fiber aids digestion, helps prevent constipation, and is sometimes used for the  treatment of diverticulosis, diabetes, and heart disease.

There are two 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.

Eating a large amount of fiber in a short period of time can cause intestinal gas (flatulence), bloating, and abdominal cramps. This usually goes away once the natural bacteria in the digestive system get used to the increase in fiber in the diet. Adding fiber gradually to the diet, instead of all at one time, can help reduce gas or diarrhea.

Too much fiber may interfere with the absorption of minerals such as iron, zinc, magnesium, and calcium. However, this effect usually does not cause too much concern because high-fiber foods are typically rich in minerals.

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

To ensure that you get enough fiber, eat a variety of foods, including:

• Cereals
• Dried beans and peas
• Fruits
• Vegetables
• Whole grains

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 non-caloric fluid a day).

Peeling can reduce the amount of fiber in fruits and vegetables. Eating fiber-containing food is beneficial, whether it is cooked or raw.

Diet - fiber; Roughage; Bulk

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Updated by: Linda Vorvick, MD, Seattle Site Coordinator, Lecturer, Pathophysiology, MEDEX Northwest Division of Physician Assistant Studies, University of Washington School of Medicine. Also reviewed by David Zieve, MD, MHA, Medical Director, A.D.A.M., Inc.