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Fiber

Today, just about every nutrition book and newsletter trumpets the praises of fiber without explaining exactly what it is and what it does. So before you dive into your next bowl of All-Bran, here's a brief look behind the labels.

To start with, dietary fiber, or roughage, is a type of carbohydrate that's found only in plant products such vegetables, nuts, fruits, and grains. A considerable body of research evidence has accumulated that links an increased fiber intake (25 to 30 grams a day) with positive effects on heart disease, cancer, diabetes, constipation, diarrhea, diverticulitis, hemorrhoids, and ulcerative colitis. With a list like that, it's no wonder we keep hearing how import fiber is for good health. Fiber is good for you!

Fiber's imposing set of talents is made even more impressive when you consider that it doesn't contribute any calories, essential vitamins, or minerals to the body. In fact, it's not digested by the body at all! How can a substance we can't even digest be so healthful? The very fact that you cannot digest it is what turns out to be its greatest asset.

Why Fiber Can't Be Digested
The foods we eat are made up of a complex combination of nutrients and nonnutrient ingredients. In order for your body to absorb and use what's in food, it first has to take the food apart piece by piece. This disassembly line is your digestive system, a 26-foot-long muscular tube.

The "workers" along the digestive tract are enzymes, which are chemicals that can break apart the proteins, carbohydrates, or fats in food. Each of the dozen or so enzymes needed to digest a typical meal can perform only one action on one nutrient. For example, one enzyme specializes in splitting big proteins into smaller pieces, but a different one is needed to complete the job.

Fiber is unique because the body lacks the right enzymes to take it apart. This means that instead of being absorbed like the other carbohydrates we eat, fiber becomes part of the bulk that travels through the small intestine into and through the large intestines, and eventually out of the body.

The word "fiber" doesn't refer to one substance; rather, it represents a variety of indigestible materials that are found in plant foods. As it travels through the body, what each type of fiber does depends on how it's built. An important distinction is whether the fiber dissolves in water, so there are two main categories of dietary fiber, insoluble and soluble, and their health benefits differ.

Insoluble Fiber
The most familiar of the insoluble fibers is wheat bran, but this type is also found in vegetables, fruits, and whole grains such as corn, rye, barley, and brown rice. Insoluble fiber increases the bulk and weight of the stool, as well as the rate at which food travels through the digestive system. This makes for potential benefits against cancer. Population studies routinely find that the incidence of colon cancer goes down as the intake of insoluble fiber goes up. That's because fiber can effectively dilute or even grab on to potential cancer-causing substances and quickly usher them out of the body. That's some bodyguard!

Insoluble fiber also gets mixed together with the bile salts, the digestive juice produced by the liver that helps with the absorption of dietary fats. Because the liver makes its bile salts from cholesterol, this binding action by insoluble fiber causes more cholesterol to leave the body.

Soluble Fiber
Perhaps the most famous soluble fiber is oat bran, but this type of fiber is also found in rice bran, legumes (beans, lentils, and peas), fruits, and vegetables. Although these fibers dissolve in water, the body cannot absorb them because of their large size. Soluble fiber can't match the ability of insoluble fiber to add bulk. It can, however, improve conditions connected with diabetes because it tends to slow the rate at which the body absorbs sugar. In addition, through a complex series of reactions, soluble fiber has a demonstrated ability to help lower blood cholesterol levels. Fruits and vegetables often contain both soluble and insoluble fibers. This is further testimony to the wisdom behind including them in the diet.

Figuring Your Fiber Strategy
At present, the typical American diet contains about 12 to 15 grams of dietary fiber per day. Most authorities recommend twice this amount, up to at least 25 to 30 grams per day. An adequate fiber intake is an essential part of a healthy diet.