Found only in plant foods, such as whole grains, fruits, vegetables, beans, nuts, and seeds, fiber is composed of complex carbohydrates. Most plant foods contain a mixture of the two types of fiber: insoluble and soluble. That is, they are composed of materials that are either soluble or insoluble in boiling water.  Insoluble fiber includes cellulose, some forms of hemicellulose, and lignin.  Soluble fiber, on the other hand, includes pectins, gum, and mucilages.

High levels of soluble fiber are found in dried beans, oats, barley, fruits, and vegetables.  Good sources of insoluble fiber are wheat bran, whole grains, cereals, seeds, and the skins of fruits and vegetables. Cooking does not decrease the amount of fiber in food; it only changes the three-dimensional structure.

Unlike other food substances, fiber is not digested; it is fermented by bacteria in the large intestine (colon). Fiber is necessary for normal bowel function, and an adequate intake can prevent constipation, diverticulitis, and hemorrhoids.

The current Recommended Dietary Allowances has not established an intake level for dietary fiber, although many health organizations recommend 20-35grams per day.  Average fiber intake by adults in the US is estimated at about 12grams per day. At this level, most Americans don't realize fiber's potential benefits. The Food and Drug Administration has emphasized dietary fiber's importance by requiring it to be listed on food labels, and has allowed the following claims to be made:

• Diets low in fat and rich in fiber-containing grain products, fruits, and vegetables may reduce the risk of some types of cancer.
• Diets low in saturated fat and cholesterol and rich in fruits, vegetables, and grain products that contain fiber, particularly soluble fiber, may reduce the risk of coronary heart disease.
• Diets low in fat and rich in fruits and vegetables, which are low-fat foods and may contain fiber or vitamin A (as beta-carotene) and vitamin C, may reduce the risk of some cancers.

Fiber's Health Benefits

Cancer:  Epidemiologic studies have consistently noted an association between total fat and high fiber intakes and reduced incidence of colon cancer.  A 1992 study by research at Harvard Medical School found that men who consumed 12 grams of fiber a day were twice as likely to develop pre-cancerous colon changes as men whose daily fiber intake was about 30 grams.  More recently, a prospective study of almost 89,000 women, published in the New England Journal of Medicine in January of 1999, failed to report an important protective effect of dietary fiber against colorectal cancer or adenoma. This information seems contradictory, but the women with the highest fiber intakes consumed about 25 grams/day, the lower end of current recommendations.  A closer look at this study, emphasizes the fact that still higher amounts of fiber in the diet may be required to promote cancer prevention. 

The exact mechanism for reducing cancer risk is not known, but scientists theorize that insoluble fiber adds bulk to stool, which in turn dilutes carcinogens and speeds their transit through the lower intestine and out of the body.

The evidence that a high-fiber diet can protect against breast cancer is equivocal. Researchers analyzing data from the Nurses' Health Study, which tracked 89,494 women for eight years, concluded in 1992 that fiber intake has no influence on breast cancer risk in middle-aged women.  Previously, a review and analysis of 12 studies found a link between high fiber intake and reduced risk.

In the early stages, some breast tumors are stimulated by excess amounts of estrogen circulating in the bloodstream.  Fiber may hamper the growth of such tumors by binding with estrogen in the intestine.  This prevents the excess estrogen from being reabsorbed into the bloodstream.

Digestive disorders:  Insoluble fiber aids digestion, adds bulk to stool,  hastens passage of fecal material through the gut, and thereby prevents or alleviates constipation.  Fiber also may help reduce the risk of diverticulosis, a condition in which small pouches form in the colon.  Symptoms include some or all of the following: constipation, diarrhea, abdominal pain, flatulence, and mucus or blood in the stool.  Increased fiber consumption can alleviate these symptoms.

Diabetes:  Soluble fiber traps carbohydrates to slow their digestion and absorption.  This may prevent wide swings in blood sugar level throughout the day.  A recent study from the Harvard School of Public Health, published in the Feb. 12 issue of the Journal of the American Medical Association, suggests that a high-sugar, low-fiber diet more than doubles women's risk of Type II (non-insulin-dependent) diabetes.  In the study, cereal fiber was associated with a 28 percent decreased risk, while fiber from fruits and vegetables had no effect.

Heart Disease: Clinical studies show that a heart-healthy diet (low in saturated fat and cholesterol, and high in fruits, vegetables, and grain products that contain soluble fiber) can lower blood cholesterol.  In these studies, cholesterol levels dropped between 0.5 percent and 2 percent for every gram of soluble fiber eaten per day.

Soluble fiber binds to dietary cholesterol in the digestive tract, aiding its elimination.  This reduces serum cholesterol levels, in turn reducing cholesterol deposits on arterial walls. There also is some evidence that soluble fiber can slow the liver's manufacture of cholesterol, as well as alter low-density lipoprotein (LDL) particles to make them larger and less dense.  Small, dense LDL particles pose a bigger health threat.

Recent findings from two long-term studies of men suggest that high fiber intake can significantly lower the risk of heart attack.  Men who ate the most fiber-rich foods (35 grams a day) suffered one-third fewer heart attacks than those who had the lowest fiber intake (15 grams a day), according to a Finnish study of 21,903 male smokers aged 50 to 69, published in the December 1996 issue of Circulation.  Earlier in the year, findings from an ongoing US study of 43,757 male health professionals (some of whom were sedentary, overweight or smokers) suggest that those who ate more than 25 grams of fiber per day had  36 percent lower risk of developing heart disease than those who consumed less than 15 grams daily.  In the Finnish study, each 10 grams of fiber added to the diet decreased the risk of dying from heart disease by 17 percent; in the US study, risk was decreased by 29 percent.

These results indicate that high-fiber diets may help blunt the effects of smoking and other risk factors for heart disease.

Obesity:  Since insoluble fiber is indigestible and passes through the body virtually intact, it provides few calories. Also, since the digestive tract can handle only so much bulk at a time, fiber-rich foods are more filling than other foods- so people tend to eat less.  Insoluble fiber may also hamper the absorption of calorie-dense dietary fat.  So, reaching for an apple instead of a bag of chips is a smart choice for someone trying to lose weight.

Potential Hazards: There are some concerns that phytate, which accompanies dietary fiber, will decrease the absorption of certain minerals like iron calcium, magnesium, and manganese.  However, most long-term studies have not confirmed this danger.  Very high intakes of isolated sources of dietary fiber can prevent adequate intake of other nutrients and may, in rare instances, lead to intestinal obstruction or volvulus of the colon.  Most untoward effects have been traced to inadequate fluid intake with consumption of large amounts of fiber, usually in a purified form.  This indicates the desirability of obtaining fiber from food sources, rather than from supplements.