The Health Effects of Fiber
The Health Effects of Fiber
Fiber is hardly the cure-all some have suggested. But neither does fiber belong at the bottom of the nutritional totem pole, where it resided for more than a century as a nonessential dietary ingredient. There is good evidence, for example, that certain dietary fibers can lower blood cholesterol levels and improve the processing of blood sugar by diabetics.
Although the evidence is conflicting, high-fiber diets have been helpful to many patients with chronic intestinal disorders, such as constipation, spastic colon, diverticular disease, and even Crohn’s disease (regional enteritis or ileitis). And there is some evidence that fiber can help to lower blood pressure and ward off gallstones.
But many of the more dramatic claims about fiber – such as its purported ability to prevent the development of intestinal disorders, hemorrhoids, appendicitis, varicose veins, obesity, colon cancer, and heart disease – are as yet untested and unproved. Furthermore, excessive consumption of fiber can be harmful, producing painful intestinal gas, flatulence, nausea, and vomiting and perhaps interfering with your body’s ability to absorb certain essential minerals (though this is not a problem if your diet is balanced and your fiber consumption moderate).
Fiber is best consumed as an integral part of foods – whole grain cereals and breads, nuts, seeds, fruits, and vegetables – rather than as a dietary supplement. Fiber “pills” are of no known benefit. The different fibrous substances have different – and sometimes even opposing – effects. Pretreatment of a fibrous food – whether, for example, it is coarsely or finely ground, whole or minced, raw or cooked – can change how it affects the person who consumes it.
Therefore, it pays to know what fiber can and cannot do, both good and bad, before you make dramatic changes in your diet. No matter what you hear or read about fiber’s benefits, people with diet-related disorders should never attempt self-treatment with fiber without first consulting their physicians.
Gastrointestinal effects. Fiber substances can absorb many times their weight in water, and most pass through the human digestive tract undigested, resulting in softer and bulkier stools.
Bran, which is mostly cellulose, is the best bet for relieving constipation. It holds a lot of water, softening the stool, increasing its volume, and making it easier to pass, and it speeds the elimination of food wastes. Apples (unpeeled), fresh carrots, and cabbage are also useful in countering constipation, as are bulk laxatives like Metamucil (hemicellulose and gum extracted from psyllium seeds).
Dr. Peter Van Soest, nutrition researcher at Cornell University, has shown that although coarse bran and cellulose have a laxative effect, finely ground bran and wood cellulose (which is also a fine fiber) induce constipation. In fact, he has found that eating bread made solely from wood cellulose kills off bacteria normally present in the gut that detoxify potentially harmful chemicals. However, Dr. Van Soest says, commercial breads made with wood cellulose also contain wheat bran and other fibers that probably counter this effect.
Weight control. Before it gets to the large intestine, fiber swells and creates a feeling of fullness that may help you cut back on the amount of food you consume and lengthen the time between meals. Young men on a diet that included twelve slices of bread a day lost more weight eating whole grain (high-fiber) bread than refined white bread.
An apple, with all its natural fiber intact, is more satisfying than applesauce, which in turn is more satisfying than apple juice, a study showed. In another study, volunteers who ate most of their carbohydrates as refined sugars often felt hungry, whereas those fed the like amount of carbohydrates as vegetable and cereal starches complained of being “stuffed.”
Many fibrous foods also take longer to chew and thus force you to eat more slowly and give your brain time to register satiety before you overconsume calories.
Diabetes. When placed on a diet high in fiber-rich carbohydrate foods but very low in fat and sugar, diabetics can greatly improve control of their blood sugar, according to studies by Dr. James W. Anderson of the Veterans Administration Hospital in Lexington, Kentucky. Many patients are able to stop using antidiabetic drugs, including insulin, or they can significantly reduce the dosage of such drugs, he reports. Pectins and gums have the most striking immediate effect on blood sugar, whereas wheat bran (cellulose) seems to offer some long-term benefits.
Dr. David J. A. Jenkins of Radcliffe Infirmary in Oxford, England, showed that fiber delays the passage of food from the stomach to the intestine. This slows the conversion of starches to the blood sugar glucose, which in turn reduces the need for a large output of insulin, the hormone that functions improperly in diabetes. This benefit of fiber is obtained only if high-fiber foods are included in a meal, not as a between-meal supplement.
Blood fats. High levels of blood fats are associated with an increased risk of heart disease, and certain fibers can reduce the amount of fatty substances – cholesterol and triglycerides – in the blood. Here again, the soluble fibers – pectins and gums – are most effective, whereas wheat bran offers no benefit.
Blood cholesterol reductions of 13 to 22 percent have been achieved simply by adding foods rich in soluble fibers, such as whole oats, oat bran, carrots, or guar crisp bread, to the diet. But the greatest improvement results from simultaneously eating less fats and cholesterol and more starchy carbohydrates. Dr. Anderson got nearly a 60 percent reduction in triglyceride levels in people with abnormally high triglycerides by putting them on a high-carbohydrate, low-fat diet rich in soluble fibers.
The risks. High-fiber diets can also have some undesirable effects. The most common is a temporary increase in flatulence and bloating, usually lasting about three weeks but in some cases for two or more months. Fibrous foods may impair the absorption of other nutrients, including iron and zinc, but this is thought to be no problem among otherwise well-nourished people.
Eating very large amounts of fiber, more than people could, can cause sigmoid volvulus, an enlargement and twisting of the sigmoid colon. Ulcerative colitis or regional ileitis may be aggravated by fiber, and people with these conditions should consult their physicians before increasing their fiber intake. To prevent blockage of the gastrointestinal tract, cereal fibers such as bran are best consumed with liquid.
Those who decide to leap on the fiber bandwagon are advised to proceed with care since the gastrointestinal tract may rebel at a sudden, large increase in fiber. The amount of added fiber to work up to is about two to four grams of crude fiber a day (Americans already eat about four grams of crude fiber daily), or about forty grams of dietary fiber.
One cup of all-bran cereal contains two grams of crude fiber. Three raw carrots or three apples a day are good alternatives. Mangoes, turnips, dried beans and peas, and leafy vegetables are other good fiber sources. Because bran has no effect on cholesterol levels, a number of experts recommend putting more emphasis on fresh fruits and vegetables than on grains.
A longtime student of dietary fiber, Dr. David Kritchevsky, associate director of the Wistar Institute in Philadelphia, concludes that increasing the fiber content of the typical American diet “can’t hurt and probably will help as long as you pay attention to whatever else you eat.
“Fiber,” he says, “is not a panacea – you can’t simply add it to a bad diet and expect to get good results.”