Can high-fiber diets really do all they claim to do? Studies have looked at the relationship between high-fiber diets and many diseases, including colon cancer, coronary heart disease and diabetes. Proven benefits of a high-fiber diet include prevention and treatment of constipation, hemorrhoids and diverticulosis. In addition, certain types of fiber help decrease blood cholesterol levels and blood glucose levels. Dietary fiber in foods slows the conversion of carbohydrates to blood glucose making the meal lower glycemic. Low glycemic means a food is slower in raising blood glucose.

What Is Dietary Fiber?

Dietary fiber comes from the portion of plants that is not digested by enzymes in the intestinal tract. Bacteria in the lower gut, however, may metabolize part of it.

Different types of plants have varying amounts and kinds of fiber, including pectin, gum, mucilage, cellulose, hemicellulose and lignin. Pectin and gum are water-soluble fibers found inside plant cells. They slow the passage of food through the intestines but do nothing to increase fecal bulk. Beans, oat bran, fruit and vegetables contain soluble fiber.

In contrast, fibers in cell walls are water insoluble. These include cellulose, hemicellulose and lignin. Such fibers increase fecal bulk and speed up the passage of food through the digestive tract. Wheat bran and whole grains contain the most insoluble fiber, but vegetables and beans also are good sources. Recently psylliun has become a popular high fiber source. Psylliun is higher in dietary fiber than most other sources.

Sometimes there is confusion as to the difference between crude fiber and dietary fiber. Both are determined by a laboratory analysis, but crude fiber is only one-seventh to one-half of total dietary fiber.

Benefits of Fiber

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.

Sources of Fiber

Dietary fiber is found only in plant foods: fruits, vegetables, nuts and grains. Meat, milk and eggs do not contain fiber. The form of food may or may not affect its fiber content. Canned and frozen fruits and vegetables contain just as much fiber as raw ones. Other types of processing, though, may reduce fiber content. Drying and crushing, for example, destroy the water-holding qualities of fiber.

The removal of seeds, peels or hulls also reduces fiber content. Whole tomatoes have more fiber than peeled tomatoes, which have more than tomato juice. Likewise, whole grain bread contains more fiber than white bread. Table 2 lists the dietary fiber content of some common foods.

How Much Fiber?

In 2002, the Food and Nutrition Board of the National Academy of Sciences Research Council issued Dietary Reference Intakes (DRI) for fiber (see Table 1). Previously, no national standardized recommendation existed. The new DRIs represent desirable intake levels established using the most recent scientific evidence available. The current recommendations range between 19 grams per day and 38 grams per day depending on age and gender. However, the average American only consumes 14 grams of dietary fiber per day.

For many people, meeting the DRI for fiber may require changes in their eating habits. Eating several servings of whole grains, fruits, vegetables and dried beans each day is good way to boost fiber intake. However, if you are not used to eating high fiber foods regularly, these changes should be made gradually to avoid problems with gas and diarrhea. Anyone with a chronic disease should consult a physician before greatly altering a diet.

Food Labeling of Fiber

Nutrients required on food labels reflect current public health concerns and coincide with current public health recommendations. Nutrition labels now list a Daily Reference Value (DRV) for specific nutrients, including fiber. The DRV for fiber is 25 grams per day based on a 2,000 calorie diet, or 30 grams per day based on a 2,500 calorie diet. The fiber content of a food is listed in grams and as a percentage of the daily value.

The illustration shows a food nutrition label. It tells you the product provides 3 g of fiber in a half-cup serving. The percent Daily Value for one serving is 12 percent, or 12 percent of DRV of 25 grams based on a 2,000 calorie diet.

Specific health claims can be made for food products that meet specific requirements. For example: "Diets low in saturated fat and cholesterol and rich in fruits, vegetables and grain products that contain fiber, particularly soluble fiber, may reduce the risk of coronary heart disease." In order to make a health claim about fiber and coronary heart disease, the food must contain at least 0.6 g of soluble fiber per reference amount. The soluble fiber content must be listed and cannot be added or fortified. A product containing a health claim for fiber and coronary heart disease must also meet the definitions of a low fat, low in saturated fat and low in cholesterol product.

A statement such as "made with oat bran" or "high in oat bran" implies that a product contains a considerable amount of the nutrient. Claims that imply a product contains a particular amount of fiber can be made only if the food actually meets the definition for "high fiber" or "good source of fiber," whichever is appropriate.

 

The following terms describe products that can help increase fiber intake:

• High fiber: 5 g or more per serving
• Good source of fiber: 2.5 g to 4.9 g per serving
• More or added fiber: At least 2.5 g more per serving than the reference food

Although fiber is important, it is just one part of a properly balanced diet. It is possible that too much fiber may reduce the amount of calcium, iron, zinc, copper and magnesium that is absorbed from foods. Deficiencies of these nutrients could result if the amount of fiber in the diet is excessive, especially in young children.

Fiber supplements are sold in a variety of forms from bran tablets to purified cellulose. Many laxatives sold as stool softeners actually are fiber supplements. Fiber's role in the diet is still being investigated. It appears that the various types of fiber have different roles in the body.

For these reasons, avoid fiber supplements. Instead, eat a variety of fiber-rich foods. This is the best way to receive the maximum benefits from each type of fiber present in foods, and obtain necessary nutrients.

References

• Farley, Dixie. May 1993. Look for 'LEGIT' Health Claims on Foods. FDA Consumer.
• Kurtzweil, Paula. May 1993. Nutrition Facts to Help Consumers Eat Smart. FDA Consumer.
• Duyff, Roberta. American Dietetic Association's 2nd Edition Complete Food and Nutrition Guide. P130-142. 2002.
• Slavin, J. "Dietary Fiber: Mechanisms or Magic in Disease Prevention?" Nutrition Today. Nov/Dec. 1990.
• Institute of Medicine. Dietary Reference Intakes for Energy, Carbohydrate, Fiber, Fat, Fatty Acids, Cholesterol, Protein, and Amino Acids (Macronutrients). National Academy Press p. 265-334. 2002.

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