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Grains, beans and other legumes, and raw or lightly processed fruits and vegetables are the best sources of dietary fiber.

What is fiber?

Dietary fibers are the parts of plants that cannot be digested by humans. In the past, fiber was called roughage or bulk. It includes the outer covering (bran) of grains, the pectin in apples and other fruits, and the skin and fibrous parts of vegetables and fruits. There are two broad categories of fiber: soluble and insoluble. Soluble fibers dissolve in water and become sticky in consistency. Examples include:

• Pectin, found in fruits, legumes, nuts, and some vegetables.
• Gums, such as guar or carrageenan, found in algae, seaweeds, and the cluster bean.
• Mucilages, found in plant seeds and certain plant secretions.

Insoluble fiber passes through the digestive tract largely unchanged except for being broken into smaller pieces by chewing. Examples include:

• Cellulose, which helps to give plant walls stability and structure; it occurs in bran, whole grains, fruits, and vegetables.
• Hemicellulose, which is found in vegetables, fruits, nuts, and grains.
• Lignin, a woody substance found in bran, fruit skins, nuts, and grains.

Why is fiber important to nutrition?

One of the most important effects of fiber is to provide bulk to help the body get rid of waste products. Fiber absorbs many times its own weight in liquid, helping to keep feces soft and bulky and to prevent constipation. Soluble fiber also has been shown to help control blood sugar levels, to change the way the body digests fats, and to lower blood cholesterol levels.

Fiber makes a person feel full even though it does not contribute calories. Thus, it is important in controlling weight. There also is some evidence that a high-fiber diet may lessen the risk of heart attack, adult diabetes, and certain cancers, but these benefits have not yet been proved.

How much fiber do I need?

There is no recommended dietary allowance (RDA) for fiber. However, nutritionists generally recommend that an average healthy adult consume 15 to 30 grams a day. The typical American diet — high in meats, fat, sugar, and processed foods — provides about 10 grams of fiber. Thus, it is safe to say that many, if not most, Americans do not consume enough fiber in their regular diets. This can be corrected by simple dietary changes and by preparing high-fiber foods in ways that help retain their fiber.

What happens if I don't get enough fiber?
In the short term, constipation is often the result of a diet lacking in fiber. The long-term health effects may be more serious. Low-fiber diets, in combination with other dietary factors such as a high-fat or high-calorie intake, have been linked to obesity, an increased risk of diverticulosis (the formation of pockets along weakened sections of the colon wall), high blood cholesterol, and colon and breast cancers.

What happens if I get too much fiber?
Excessive fiber intake can cause diarrhea, intestinal gas, bloating, abdominal cramps, and, in unusual cases, intestinal obstruction. Excessive bran and other insoluble fibers can prevent proper absorption of iron, zinc, and other important minerals.

In extreme cases, over­loading the body with fiber can lead to an intestinal blockage that may require surgery. This most often happens to older people whose bowel function is already sluggish. In these people, suddenly eating large amounts of bran, for example, can lead to an impacted colon.

Should I take a fiber supplement?
Fiber supplements, usually in the form of pills or powders, are highly controversial. A diet that provides adequate fiber from a variety of foods also provides essential vitamins and minerals that are lacking in fiber pills. In addition, supplements do not promote improved dietary habits, and excessive use can result in malnutrition due to the malabsorption of certain nutrients, especially iron and zinc.

There are a number of different sources of fiber with different health benefits. Thus, it is important that fiber comes from a variety of foods, which also guards against other nutritional deficiencies.