February 1998 -- Nutrition Notes
By: Andrea Platzman, R.D.
Formerly called "roughage" or "bulk," fiber was once considered only a filler. It was thought that if an individual ate high-fiber foods, he would have less room for high-fat, high-calorie items. This belief has proven true, but fiber offers other benefits as well. Not only is fiber generally found in foods rich in vitamins and minerals, but scientists now recognize it might play a role in helping reduce some of the leading chronic diseases, including cardiovascular disease and cancer, diabetes and intestinal maladies such as diverticulitis.
Fiber is not a single substance, but instead encompasses a large group of different compounds with varied effects on the body and in food products. What all types of fiber share is that they are the parts of plants that cannot be digested by enzymes in the intestinal tract. Fiber can be classified into two basic categories: those that are insoluble in water and those that are soluble. Most foods contain both types in varying amounts, but certain foods are particularly rich in one or the other.
Insoluble fiber the primary type found in wheat bran, whole-grain breads, vegetables and cereals contains cellulose, hemicellulose and lignin. This type of fiber accelerates intestinal transit, increases fecal bulk and slows starch hydrolysis, thereby helping prevent diverticulitis, hemorrhoids, constipation and certain cancers. Certain fiber components, primarily hemicelluloses, absorb water and swell. This swelling produces a softer stool that moves easily through the intestinal tract, and prevents constipation. The exception is lignin, which does not absorb water.
Digestion and absorption of fat require bile acids, some of which have been implicated in colon cancer. The bulk produced by insoluble fiber binds and dilutes these acids, thereby lowering the cancer risk. Additionally, moving foods faster through the digestive tract helps reduce the exposure of potential carcinogens to the colon walls. With a low-fiber diet, food can take two to three times longer to pass through the body.
Breast cancer also might be reduced by consuming a diet high in insoluble fiber. Foods rich in insoluble fiber are usually low in fat and calories two other factors linked to reducing the onset of cancer. In addition, wheat bran reduces blood estrogen levels, which greatly affect the onset risk of breast cancer.
Fiber provides bulk. This not only contributes to general gastrointestinal health and prevents certain cancers, but aids in weight loss. By building bulk in the stomach, fiber contributes a feeling of fullness, helping curb appetite.
Soluble fiber, such as pectin, gums and mucilages, has proven helpful in treating diabetes and hypercholestemia (high cholesterol). Research indicates that adding one or two servings per day of beans, oats, psyllium husk or other sources of soluble fiber, can help lower fasting blood-sugar levels of people with diabetes. Some have even been able to reduce the amount of insulin or oral hypoglycemics needed.
By forming a viscous gel, soluble fibers can delay glucose absorption. To date, guar gum has been found to form the most viscous gel. This gel impairs the cell's uptake of glucose and water. Additionally, soluble fiber is associated with slower gastric emptying, which also affects the glycemic response. People with diabetes are recommended to consume 40 grams of fiber or 25 grams per 1,000 calories per day. This amount is higher than for the general population.
Some recent studies indicate that a diet rich in soluble fiber, particularly oligofructose and inulin, can reduce the development of colon and rectal cancers. These naturally occurring chicory fructans have been shown to stimulate the growth of bifidobacteria good bacteria which prevents carcinogenesis in the colon.
Where soluble fiber has really made its mark is in the oat bran domain. Oat bran is rich in a gum known as beta-glucan, which has been linked to cholesterol-lowering benefits. In fact, labels for oat-based products containing 0.75 gram or more of beta-glucan may carry the claim: "may reduce the risk of heart disease." Beans and psyllium fiber also have been linked to reduced cholesterol levels.
Experts believe that soluble fiber reduces cholesterol levels beginning during digestion. At this point, the liver releases cholesterol-rich bile acids into the intestine to help absorb fat. Soluble fiber forms a gel in the small intestine, surrounding the bile acids, preventing their absorption. As a result, the liver produces more bile acids. This requires the use of cholesterol, particularly low density lipoprotein (LDL) the "bad" cholesterol causing the levels of LDL to drop. In addition, recent research discovered that eating oat products does not change or, in some cases, increases levels of high density lipoprotein ("good" cholesterol).
"Though most foods do not contain a lot of fiber the average for fruits and vegetables is 1.5 grams per serving and refined grains only 1 gram per serving it would not be that difficult to consume more fiber, especially if more whole grains (2 to 5 grams per serving) and legumes (3 to 6 grams per serving) were consumed," says Judith Marlett, Ph.D., R.D., professor, department of nutrition sciences, the University of Wisconsin, Madison. "Additionally, if fiber is added to commercially prepared products, just note that oat bran is tastier than wheat bran."
According to FDA, and based on the Daily Reference Values (DRV), a "good source of fiber" contains at least 10% of the DRV for fiber, or 2.5 grams. A "high source of fiber" contains at least 20% of the DRV for fiber, or 5 grams. Additionally, these products also must be low in total fat.
"Although a high-fiber diet has many benefits, most people get about half of the recommended 25 to 30 grams each day," observes Joanne Slavin, Ph.D., R.D., professor, department of food science and nutrition, the University of Minnesota, St. Paul. "The main barrier for the people to get enough daily fiber is that the foods they enjoy most are not high in fiber."
Whether you get your fiber from naturally occurring sources or from fortified food products doesn't really matter. The bottom line to reaping the benefits is consuming a fiber-rich diet.
Andrea D. Platzman is a registered dietitian who is a consultant to the food industry, and regularly writes for nutrition publications. She earned a master's degree in nutrition from New York University, and has a strong culinary and business background.
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