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.

Dietary fiber comes from the portion of plants that is not digested by enzymes in the intestinal tract. Part of it, however, may be metabolized by bacteria in the lower gut. 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.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.

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.It is now known that a high-fiber diet gives better results once the inflammation has subsided.

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.

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. 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 wheat bread contains more fiber than white bread.

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.