Dietary fiber is a group of complex carbohydrates that are not a source of energy for human beings. Because human digestive enzymes cannot break the bonds that hold fiber’s sugar units together, fiber adds no calories to your diet and cannot be converted to glucose.

Ruminants (animals, such as cows, that chew the cud) have a combination of digestive enzymes and digestive microbes that enable them to extract the nutrients from insoluble dietary fiber (cellulose and some hemicelluloses).

But not even these creatures can pull nutrients out of lignin, an insoluble fiber in plant stems and leaves and the predominant fiber in wood. As a result, the U.S. Department of Agriculture specifically prohibits the use of wood or sawdust in animal feed.

But just because you can’t digest dietary fiber doesn’t mean it isn’t a valuable part of your diet. The opposite is true. Dietary fiber is valuable because you can’t digest it!

Two kinds of dietary fiber

Nutritionists classify dietary fiber as either insoluble fiber or soluble fiber, depending on whether it dissolves in water. (Both kinds of fiber resist human digestive enzymes.)

• Insoluble fiber: This type of fiber includes cellulose, some hemicelluloses, and lignin found in whole grains and other plants. This kind of dietary fiber is a natural laxative.

It absorbs water, helps you feel full after eating, and stimulates your intestinal walls to contract and relax. These natural contractions, called peristalsis, move solid materials through your digestive tract.

By moving food quickly through your intestines, insoluble fiber may help relieve or prevent digestive disorders such as constipation or diverticulitis (infection that occurs when food gets stuck in small pouches in the wall of the colon).

Insoluble fiber also bulks up stool and makes it softer, reducing your risk of developing hemorrhoids and lessening the discomfort if you already have them.

• Soluble fiber: This fiber, such as pectins in apples and beta-glucans in oats and barley, seems to lower the amount of cholesterol circulating in your blood (your cholesterol level). This tendency may be why a diet rich in fiber appears to offer some protection against heart disease.

Here’s a benefit for dieters: Soluble fiber forms gels in the presence of water, which is what happens when apples and oat bran reach your digestive tract. Like insoluble fiber, soluble fiber can make you feel full without adding calories.

Ordinary soluble dietary fiber can’t be digested, so your body doesn’t absorb it. But in 2002, researchers at Detroit’s Barbara Ann Karamonos Cancer Institute fed laboratory mice a form of soluble dietary fiber called modified citrus pectin. The fiber, which is made from citrus fruit peel, can be digested.

When fed to laboratory rats, it appeared to reduce the size of tumors caused by implanted human breast and colon cancer cells. The researchers believe that the fiber prevents cancer cells from linking together to form tumors.

Now, two pharmaceutical companies — one on the East Coast and one on the West Coast — are investigating the effects of modified citrus pectin in human beings.

But the product isn’t yet ready for prime time. Although it’s being sold as a dietary supplement (not as a medicine), experts warn that its effects on human bodies (and human cancers) remain unproven.

Getting fiber from food

You find fiber in all plant foods — fruits, vegetables, and grains. But you find absolutely no fiber in foods from animals: meat, fish, poultry, milk, milk products, and eggs. A balanced diet with lots of foods from plants gives you both insoluble and soluble fiber.

Most foods that contain fiber have both kinds, although the balance usually tilts toward one or the other. For example, the predominant fiber in an apple is pectin (a soluble fiber), but an apple peel also has some cellulose, hemicellulose, and lignin.

Table 8-1 shows you which foods are particularly good sources of specific kinds of fiber. A diet rich in plant foods (fruits, vegetables, grains) gives you adequate amounts of dietary fiber.

Fiber	Where Found
Soluble fiber
Pectin	Fruits (apples, strawberries, citrus fruits)
Beta-glucans	Oats, barley
Gums	Beans, cereals (oats, rice, barley), seeds, seaweed
Insoluble fiber
Cellulose	Leaves (cabbage), roots (carrots, beets), bran, whole wheat, beans
Hemicellulose	Seed coverings (bran, whole grains)
Lignin	Plant stems, leaves, and skin