Dietary Fiber: The Non-Nutrient in Carbohydrates
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.)
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.
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.
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!
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 (infection that occurs when food gets stuck in small pouches in the wall of the colon). 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.
The following table shows you which foods are particularly good sources of specific kinds of fiber. A diet rich in plant foods (fruits, vegetables, grains) gives you adequate amounts of dietary fiber.
|Pectin||Fruits (apples, strawberries, citrus fruits)|
|Gums||Beans, cereals (oats, rice, barley), seeds, seaweed|
|Cellulose||Leaves (cabbage), roots (carrots, beets), bran, whole wheat, beans|
|Hemicellulose||Seed coverings (bran, whole grains)|
|Lignin||Plant stems, leaves, and skin|