Upbeat on Fiber for Longer Life and Better Health

In the 1800s, Sylvester W. Graham joined fame espousing the benefits of roughage (Fiber) in the diet. His cracker is still with us, but surprisingly, dietary fiber has had a harder time staying in favor. Since the turn of the century, the fiber intake of Americans has steadily declined.
But today dietary fiber is making a comeback, as studies have multiplied linking it to a lower risk for some of the leading causes of death in America. Just what is this food component, and why do the experts tell us to eat more of it?

What is Fiber?

Dietary fiber generally refers to parts of fruits, vegetables, grains, nuts and legumes that can't be digested by humans. Meats and dairy products do not contain fiber.
There are two basic types of fiber -- insoluble and soluble. Most fiber-containing foods feature both, but one or the other type often predominates in specific parts of a food and determines the characteristic texture of that portion of the food.
For example, insoluble fibers produce the tough, chewy feel of wheat kernels, popcorn, apple skin and nuts. Essential to the cellular structure of plants, insoluble fibers include cellulose, hemicelluloses and ligin. They do not dissolve in water.
Soluble fibers include pectin, gums, mucilages and algal polysaccharides. Although pectin is part of cell walls, most soluble fibers are found within plant cells. The gummy essence of oat bran and the mushy center of a cooked kidney bean reflect both the soluble fiber content of those foods and the ability of soluble fibers to soak up water.
The fiber content of a food varies according to the species of the plant and stage of maturation, but seeds, berries, fruit skins and the bran layers of cereal grains generally contain larger amounts of a plant's fiber.

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Fiber and Health

Although it wasn't called "fiber" until the 1950s, Hippocrates realized the laxative effects of dietary fiber in 430 B.C.. Not until the 1960s, however, did scientists seriously begin to investigate the role of fiber in health.
At that time, studies showed rural Africans, with diets higher in fiber than the typical American or European diet, had a lower incidence of colon cancer, diverticulosis, hemorrhoids, gallstones, appendicitis, diabetes and some forms of heart disease than Americans or Europeans.
Studies since generally have indicated a positive relationship between a high-fiber diet and good health, although it has been difficult to separate the effects of fiber from other dietary and lifestyle factors that may play a role in health.
"It's the total dietary pattern that has been linked to a reduced risk of disease," says Bruce Trock, Ph.D., cancer epidemiologist at the Fox Chase Cancer Center in Philadelphia. "Looking at all the studies together, what you find is that a diet that's high in grains, vegetables and fruits -- which also is a diet that's high in fiber -- is clearly protective against colon cancer and possible cardiovascular diseases."
A high-fiber diet appears to reduce disease risk by increasing fecal bulk, decreasing the transit time of food through the gastrointestinal tract, reducing blood cholesterol levels and helping to control blood sugar levels. With their distinct physical characteristics, insoluble and soluble fibers work differently to produce these results.
Insoluble fibers seem to have their greatest impact on the health of the colon or large intestine. Large amounts of insoluble fibers increase fecal bulk and draw water into the large intestine. The result is a larger, softer stool that exerts less pressure on the colon walls and is eliminated more quickly. Indeed, the most well-established benefit of a high-fiber diet is in the treatment and prevention of constipation. The reduced pressure also may help prevent diverticulosis (small herniations in the colon wall that may become inflamed). In addition, large amounts of insoluble fibers dilute the concentration of potential carcinogens that may be present in the stool, and the decreased transit time reduces the exposure of the intestinal wall to those substances. Furthermore, insoluble fibers alter the pH of the large intestine, interfering with microbial activity that produces carcinogens. The combined effect may be a reduced risk of colon cancer.
Complementing the action of insoluble fibers, some soluble fibers also add to fecal bulk and increase its water content. But soluble fiber's potential for reducing blood cholesterol levels recently has grabbed the spotlight. Studies have shown that diets rich in soluble fibers such as oat bran may help reduce total cholesterol and low density lipoprotein (LDL) cholesterol in people with both high and normal blood cholesterol levels.
Margo Denke, M.D., a nutrition research scientist at the University of Texas Southwestern Medical Center, estimates that a diet low in fat and high in soluble fiber may reduce an individual's blood cholesterol by 3 percent to 6 percent.
Soluble fibers appear to reduce blood cholesterol in two ways. First, they prevent the reabsorption of vital bile acids from the small intestine. To replace the lost bile acids, cholesterol is drawn from the body, thereby reducing its cholesterol supply. Second, the fermentation of soluble fibers in the intestine produces short-chain fatty acids which block the synthesis of cholesterol.
Studies suggest soluble fibers also may help control the rise in blood sugar following a meal and reduce insulin requirements in some patients with diabetes mellitus. By increasing the viscosity of gastrointestinal contents, soluble fibers retard gastric emptying, slowing the absorption of glucose in the process.

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Measuring Fiber

Most food composition data bases today reflect the crude fiber content of food, which is determined by subjecting food to a chemical treatment that destroys large amounts of its insoluble fibers and almost all of its soluble fibers. The resulting value seriously underestimates the true dietary fiber content of food, which has been judged to be three to five times higher. This created difficulties when attempting to evaluate fiber intake.
Methods that provide a more accurate reading are now in limited use in the United States. The neutral detergent fiber method is primarily useful in estimating insoluble fiber content. The total dietary fiber (TDF) method is the only process that estimates total fiber content. The Association of Official Analytical Chemists, which establishes all accepted U.S. methods for determining nutrients in foods, advocates use of the TDF method.
Modification of the TDF method also allows determination of soluble and insoluble fractions, but provisional data bases using figures derived from this method currently do not reflect the type of fiber in a food. According to the U.S. Department of Agriculture (USDA) Human Nutrition Information Service, an update of the information is expected in the near future.

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Dietary Fiber Recommendations

While Americans currently consume an average of 11 grams of dietary fiber daily, the National Cancer Institute advises an increase to 20 to 35 grams a day. Although soluble fibers have received much attention lately, Trock cautions against undue emphasis on any one type of fiber when planning a daily diet. "People should increase the level of fiber by increasing foods from all the vegetable, grain and fruit sources," he said.
Indeed, dietary guidelines issued by the USDA, the U.S. Department of Health and Human Services, and the Surgeon General's office emphasize an increased intake of fiber-rich foods in general. Fiber supplements are not recommended as a way to meet dietary guidelines.
The Diet and Health report of the National Academy of Sciences has gone one step further by specifying recommended amounts of foods high in fiber. It advises a daily intake of five or more servings of fruits and vegetables and six or more servings of whole grain breads and cereals and legumes.
Health professionals caution against making an immediate leap from a low-fiber intake to recommended levels. Increasing fiber consumption too rapidly can result in flatulence, cramping and intestinal distention. Undesirable side effects may be avoided through the gradual addition of fiber to the diet along with an adequate fluid intake.
Although concerns that fiber may interfere with the absorption of trace minerals have been voiced, studies show people consuming well-balanced and varied diets high in fiber are unlikely to experience mineral deficiencies.
While fiber does seem significant to human health, scientists stress its interplay with other factors must not be ignored. The effects of heredity, the quality of the overall diet and habits such as smoking and exercise can outweigh any single dietary modification. The best guarantee is an integrated effort that includes a high-fiber diet as part of a healthy lifestyle.

Reprinted from the International Food Information Council Foundation, 1990