What Is Dietary Fiber?
Dietary fiber comes from the portion of plants that is not digested by enzymes in the intestinal tract. Different types of plants have varying amounts and kinds of fiber. 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. 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 are good sources, too.
Psyllium, also known by the brand name Metamucil, is a popular over-the-counter source of fiber. Psyllium is higher in dietary fiber than most other sources. It is broken down in the large bowel and becomes a food source for the bacteria that live in the colon. These healthy bacteria bulk up the stool, creating larger, softer stools that are easier to pass. Natural soluble fiber sources include oats, oat bran and beans. Psyllium is a good alternative, but eating a variety of fiber-rich foods is the best way to receive the maximum benefits from each type of fiber and obtain necessary nutrients. Always discuss psyllium use with your health care provider. New users should start slow to prevent bloating and gas.
Benefits of Fiber
Insoluble fiber binds water, making stools softer and bulkier. Therefore, fiber—especially those found in whole grain products—is helpful in the treatment and prevention of constipation, hemorrhoids and diverticulosis. Fiber has also been found to help maintain lower blood glucose levels in diabetes. Low blood cholesterol levels (below 200 mg/dl) have been associated with a reduced risk of coronary heart disease. Because the body eliminates cholesterol through the excretion of bile acids, and water-soluble fiber binds bile acids, it has been suggested 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 insoluble fiber increasing the rate at which wastes are removed from the body. This means the body may have less exposure to toxic substances produced during digestion.
High-fiber diets may be useful for people who wish to lose weight. Fiber has no calories, yet it 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 number of calories. Foods high in fiber often require more chewing, so a person is unable to eat a high 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. 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.
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 below). Previously, no national standardized recommendation existed. The new DRIs represent desirable intake levels established using the most recent scientific evidence available. The average American only consumes 14 grams of dietary fiber per day.
Dietary Reference Intakes (DRI) for Fiber
|Males||Grams per day||Females||Grams per day|
|9-13 years||26||9-13 years||31|
|14-18 years||26||14-18 years||38|
|19-50 years||25||19-50 years||38|
|51+ years||21||51+ years||30|
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.
Specific health claims can be made for food products that meet specific requirements. 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 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,” as appropriate.
The following terms describe products that can help increase fiber intake:
High fiber: 5g or more per serving
Good source of fiber: 2.5g to 4.9g per serving
More or added fiber: At least 2.5g more per serving than the reference (or unfortified) food
Remember, 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.
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