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Can fiber help you lose weight? Does it cure chronic constipation, prevent gastrointestinal disease, lower cholesterol or halt the spread of colon cancer? These are just a handful of the questions combed over by scientists exploring the link between fiber and health. Despite more than two decades of research, firm answers have proven rare.

One stumbling block lies in difficulties in comparing evidence from various studies, where different methods may be used to assess both fiber intake and fiber content of foods consumed. Fiber itself is a catchall word for various types or "fractions" of indigestible plant food. Twenty years after the "fiber hypothesis" was first proposed, we still lack a universal definition of the term. The nub of the problem is that fiber is a complex mix of cellulose, hemicellulose, pectins, gums, mucilages, polysaccharides from algae and lignin, a non-starch.Many of the observed effects of fiber relate to the solubility of different fractions - whether they dissolve in water or not. Most structural fibers - including cellulose, lignin and some hemicellulose - are insoluble. Pectins, gums, mucilages and some hemicelluloses are soluble. Most plant foods contain both soluble and insoluble fibers, although they usually are better sources of one than of the other.

Vegetables and grains are good sources of cellulose, and bran cereals and other whole grains are the best sources of hemicelluloses. Legumes and oats are richest in gums, while apples and citrus fruits are highest in pectins.

We usually think of fiber in terms of its effects on the lower bowel, but in truth its influence begins in the mouth. High-fiber foods, which require more chewing, stimulate the flow of saliva and the secretion of gastric juices. Water-soluble fibers, such as pectins, form gels and increase the viscosity and stickiness of the stomach contents.

For some reason, soluble fibers also appear to slow the rate of digestion of food and the absorption of nutrients. Finally, as has been known for many years, dietary fiber enlarges the bulk of the feces and speeds its movement down the passageway of the bowel.

Most fibers contribute to boosting stool volume and weight through an increase in undigested and non-fermentable material, water, and/or bacterial cells and gas production by fermentable fibers. Exactly how much water a fiber holds varies considerably. Fine bran holds less water than coarse bran. And grains generally are more effective than fruits and vegetables for bulking up stool and preventing constipation.

The number of bowel movements considered normal varies from as many as three a day to as few as three a week, and the effect of increasing your fiber intake varies according to your normal bowel pattern. Usually fiber increases frequency in individuals with fewer movements and has the opposite result in those who move their bowels more often.

What can be said about the health benefits of fiber? Experiments have shown that soluble fiber fractions found in fresh fruits and vegetables, legumes, oat bran and barley can lower levels of serum cholesterol. Why, we don't know. On the other hand, insoluble fibers, like that in wheat bran, have no effect on serum cholesterol.

As for cancer of the colon, population studies suggest that a high-fiber diet is protective, while a high-fat diet may raise risk. But the combined body of evidence from human studies and animal experiments is insufficient to allow for recommendations about dietary fiber and cancer.

There has been no lack of popular claims for fiber as a weight-control aid. However, scientific studies that attempt to confirm a role for fiber in either weight-control diets or weight maintenance are inconclusive. Most important to dieters should be the firm evidence that high fiber consumption, especially wheat fiber, can prevent constipation, frequently a problem when food intake is reduced.

The role of fiber in relieving constipation, something our grandparents recognized long ago, remains the most solid of the fiber-and-health links. Closely associated with this, fiber may relieve the symptoms of diverticular disease of the bowel. This common condition may result from years of small, hard stools and strained bowel movements that increase the pressure in the bowel.

In diabetics, water-soluble viscous fibers in pectins and gums seem to help control blood sugar. But these observations should not overshadow the fact that the basic management of diabetes remains a well-planned diet and weight loss if necessary.

Until research provides a host of more specific answers, experts agree that the best approach to fiber is through eating a balanced diet, including a varied spread of fiber-rich foods.