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!

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. 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.

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.

How much fiber do you need?

According to the U.S. Department of Agriculture, the average American woman gets about 12 grams of fiber a day from food; the average American man, 17 grams. Those figures are well below the new IOM (Institute of Medicine) recommendations that I conveniently list here:

• 25 grams a day for women younger than 50.
• 38 grams a day for men younger than 50.
• 21 grams a day for women older than 50.
• 30 grams a day for men older than 50.

The amounts of dietary fiber recommended by IOM are believed to give you the benefits you want without causing fiber-related — um — unpleasantries. Unpleasantries? Like what? And how will you know if you’ve got them? Trust me: If you eat more than enough fiber, your body will tell you right away.

All that roughage may irritate your intestinal tract, which will issue an unmistakable protest in the form of intestinal gas or diarrhea. In extreme cases, if you don’t drink enough liquids to moisten and soften the fiber you eat so that it easily slides through your digestive tract, the dietary fiber may form a mass that can end up as an intestinal obstruction.

If you decide to up the amount of fiber in your diet, follow this advice:

• Do so very gradually, a little bit more every day. That way you’re less likely to experience intestinal distress. In other words, if your current diet is heavy on no-fiber foods such as meat, fish, poultry, eggs, milk, and cheese, and low-fiber foods such as white bread and white rice, don’t load up on bran cereal (35 grams dietary fiber per 3.5-ounce serving) or dried figs (9.3 grams per serving) all at once.

Start by adding a serving of cornflakes (2.0 grams dietary fiber) at breakfast, maybe an apple (2.8 grams) at lunch, a pear (2.6 grams) at mid-afternoon, and a half cup of baked beans (7.7 grams) at dinner. Four simple additions, and already you’re up to 15 grams dietary fiber.

• Always check the nutrition label whenever you shop. When choosing between similar products, just take the one with the higher fiber content per serving. For example, white pita bread generally has about 1.6 grams dietary fiber per serving. Whole wheat pita bread has 7.4 grams. From a fiber standpoint, you know which works better for your body. Go for it!

• Get enough liquids. Dietary fiber is like a sponge. It sops up liquid, so increasing your fiber intake may deprive your cells of the water they need to perform their daily work. That’s why the American Academy of Family Physicians (among others) suggests checking to make sure you get plenty fluids when you consume more fiber.