The average American diet contains about ten to thirteen grams of fiber per day, which is half of the recommended twenty to thirty-five grams per day. Diets high in dietary fiber are associated with lower risks of colon cancer, diabetes, obesity, and bowel conditions such as diverticulosis and constipation. The role that fiber has in the prevention of these conditions varies with each one, dependent on the type of fiber consumed.

Types of Dietary Fiber
There are two types of dietary fiber: water-soluble and water insoluble. Water-soluble fibers are soluble in water and the beneficial effects occur largely in the small intestine. One of the effects of water-soluble fiber is the slowing of transit time (the time it takes for contents of digestion to pass through the gastrointestinal tract) through the stomach as well as the small intestine. This causes a decrease in absorptive rates of nutrients from the small intestine and subsequent lowering of blood levels of these nutrients after a meal. This effect is particularly beneficial for the treatment of type 2 diabetes. Blood glucose and insulin levels are lower after consuming a meal containing water-soluble fiber.

The slowing of transit time through the stomach results in delayed emptying of the stomach, enabling us to feel full for a longer period of time. Research has shown that individuals who consume diets containing greater amounts of water-soluble fiber are less likely to be overweight or obese as a result of this effect.

Consumption of water-soluble fiber is also associated with lowering blood cholesterol levels. Bile acids (by-products of fat digestion) are reabsorbed in the later part of the small intestine and from there are transported to the liver and are used in the production of cholesterol. Water-soluble fibers bind up bile acids in the lower portion of the small intestine, thus decreasing their absorption and subsequently decreasing the synthesis of cholesterol.

Dietary Sources of Water-Soluble Fiber
About twenty-five to thirty-three percent of the fibers consumed in the American diet are water-soluble fibers. Foods high in this fiber include oat bran, oatmeal, kidney beans, oranges, carrots, and broccoli. Consuming the recommended five servings of fruits and vegetables daily will significantly boost your consumption of water-soluble fiber. Fiber supplements containing psyllium (Metamucil) are also a good source of water-soluble fiber.

Water-Insoluble Fiber
The effects of water-insoluble fiber are largely confined to the large intestine. These fibers bind water, which results in increased bulk of the feces and faster transit time through the large intestine (a laxative effect). This may help in the prevention of diverticulosis, hemorrhoids, and colon cancer.

The bacteria in the large intestine break down most of these fibers. Two by-products of this breakdown are carbon dioxide and methane, which is the "gas" produced after eating foods containing this fiber. This effect is pronounced if you normally do not consume this type of fiber. If you begin to consume this fiber regularly, your intestinal cells will adapt and gas production will decrease.

Another by-product of insoluble fiber breakdown is short chain fatty acids. Short chain fatty acids may promote growth and healing of the cells of the large intestine, which may also contribute to the prevention of colon cancer.

Dietary Sources of Water-Insoluble Fiber
About seventy-five percent of the fiber consumed in the American diet is in the form of water insoluble fiber. The best sources include whole grain breads, wheat bran cereal, brown rice, lentils, kidney beans, fruits, and vegetables.

To assure that you are choosing a whole wheat bread, read the list of ingredients on the food label. The first ingredient listed should be "whole wheat." Many of the breads on our supermarket shelves are brown in color and based on the name of the bread appear to be healthier. The fact is that many of them are highly refined and, as a result, are missing a lot of the nutrients, fiber, and phytochemicals (compounds found naturally in plants that bind up oxygen-containing molecules that damage cells) found in breads made with the whole wheat. Choose a whole grain bread that contains at least two grams of dietary fiber per slice. Label reading is essential to assuring that your diet is adequate in dietary fiber.

What About Other Foods?
Like breads, many of the starches consumed in the American diet are highly refined. Examples include white breads, pasta, white rice, crackers, and cereals. To boost fiber intake, substitute these products with the whole grain version of these foods. Examples include brown rice, whole-wheat crackers, and whole grain cereals. Choose a cereal that contains at least five grams of dietary fiber per serving and a starch that contains at least two grams of dietary fiber per serving. Most highly refined starches contain little or no dietary fiber. In summary, the best way to boost your dietary fiber intake is to consume at least five servings per day of fruits and vegetables, choose whole grain and whole wheat breads, cereals and starches, and consume at least two to three servings per week of beans, peas, or lentils.

Denise Feeley is an avid runner and Registered Dietitian with more than seven years of experience counseling athletes. She is currently an adjunct professor in the Department of Exercise Science at George Washington University. She is available for nutrition counseling. For information or questions contact her at denfeeley@aol.com.