John Harvey Kellogg developed cornflakes to provide more “roughage” for the American diet. For years, cereals and other high-fiber foods have been promoted as healthful. And from time to time, the public has gone through waves of enthusiasm for increasing fiber in the diet.
In recent years, advertising for Kellogg’s All-Bran has stoked the fiber furor with a vengeance. Kellogg’s TV commercials have said explicitly that a high-fiber diet (and by implication, a diet featuring bran cereals) “may help to prevent certain kinds of cancer.” The ads are correct—as far as they go. They would be more correct if the actor who delivers that line put a heavy stress on the word may.
Dietary fiber, or roughage, is the indigestible part of plant foods—fruits, vegetables, grains, and legumes. It adds bulk to the feces and may help to prevent constipation, hemorrhoids, and diverticulitis (a common, and occasionally serious, intestinal disorder). But those effects aren’t what’s making fiber famous. Rather, it’s fiber’s reported potential for reducing the risk of cancer of the colon.
Among all cancers, colon cancer is second only to lung cancer in its toll of American lives. Every year, the disease kills about 60,000 people. Every year, physicians diagnose some 90,000 new cases. Only about half of those diagnosed survive five or more years after diagnosis. So there’s good reason indeed to pay attention to dietary measures that might reduce the risk of the disease.
In 1984, the U.S. Department of Health and Human Services said that if Americans ate less fat and more fiber, cases of colon cancer in the United States could fall by 30 percent, saving some 20,000 lives each year. The Canadian government, the U.S. Department of Agriculture, and the National Cancer Institute, among others, have also suggested that people should eat more fiber.
The official endorsements of fiber were quickly seized upon by product marketers. Store shelves are now stacked high with high-fiber breads, breakfast cereals, and packages of wheat, corn, and oat bran. The Kellogg’s All-Bran campaign even boasts an implicit endorsement by the National Cancer Institute.
The Anatomy of Fiber
Fiber comes from plants—in different amounts from different parts of different plants. A grain of wheat, for instance, contains about 12 percent fiber overall. But the grain’s endosperm, the part millers grind to make white flour, is less than 4 percent fiber. The wheat grain’s germ is about 13 percent fiber. And the bran that covers the outside of the wheat grain is more than 40 percent fiber. Whole-grain breads and bran cereals thus provide a great deal more dietary fiber than does white bread.
Further, dietary fiber consists of a number of different substances, which appear to affect the body in different ways. These substances can be divided into two types, soluble and insoluble. Soluble fiber dissolves in hot water; insoluble fiber does not. Some plant foods contain mostly soluble fiber, others mainly insoluble fiber. But virtually every food is a mixture; almost no foods provide a “pure form” of one type or the other.
Soluble fiber (pectin, gums, and some other substances) appears to have several beneficial effects. It adds bulk and thickness to the contents of the stomach and may slow emptying of the stomach, thus prolonging the sense of fullness and possibly helping dieters control their appetites. Studies have shown that soluble fiber lowers blood cholesterol levels somewhat. It also slows the absorption of sugars from the small intestine, which may be of benefit to diabetics.
Soluble fiber is readily available from a wide variety of grains, fruits, and vegetables. Good sources are prunes, pears, oranges, apples, dry beans, cauliflower, zucchini, sweet potatoes, and oat and corn bran.
However, there is little credible evidence that soluble fiber helps to prevent colon cancer. The evidence that exists points to insoluble fiber for possible protection.
Insoluble fiber includes cellulose, lignin, and hemicellulose (the last is also available as soluble fiber). The best sources of insoluble fiber are whole-grain cereals, especially wheat-bran cereals, and whole-grain breads.
Insoluble fiber adds bulk to the contents of the intestine, rather than the stomach. That speeds the passage—the “transit time”—of a meal’s remnants through the small and large intestines. (That’s why insoluble fiber helps prevent constipation.) The speedier passage through the colon may decrease the time that cells in the colon’s lining are exposed to toxins—including carcinogens—that could be present, thus reducing the chances of colon cancer. Researchers have also proposed several other theories for the presumed protective effect of insoluble fiber. For example:
- Insoluble fiber may inactivate certain carcinogens or interfere with their effect.
- Insoluble fiber may lessen the production of bacterial enzymes that can convert bile acids into carcinogens.
- Insoluble fiber stimulates the secretion of mucus in the colon. Mucus coats the colon wall and may provide a barrier that keeps toxic substances, including carcinogens, from reaching the colon’s cells.
The Evidence on Colon Cancer
When CU wrote in 1981 about reports that a high-fiber diet may help to prevent colon cancer, we regarded it as a theory still far from proven. Unproven it remains. But in the years since then, evidence has continued to accumulate suggesting that appropriate amounts of fiber may have such a protective effect. Here’s a look at what the evidence to date shows, and why it falls short of being conclusive.
Most of the evidence for the anticancer effect of fiber comes from epidemiological studies—that is, studies of diseases as they appear in populations of people. The first hint that fiber could protect against cancer came to public attention in 1974, when Denis P. Burkitt, a British physician, and his colleagues reported that rural Africans, who suffer much less colon cancer than Americans, typically consume much more fiber (50 to 150 grams per day) than do Americans (10 to 20 grams per day).
In another study, researchers found that Danes in Copenhagen, who consumed an average of 17 grams of fiber per day, suffered from three times as much colon cancer as the Finns of Kuopio, who consumed an average of 31 grams per day. Similar correlations have been shown in studies in Great Britain, Connecticut, and elsewhere, but not consistently. In general, the key difference between the populations under study appeared to be in their consumption of insoluble fiber found in whole-grain cereals and whole-grain baked goods.
Also, people on vegetarian or semivegetarian diets—which often contain high levels of both soluble and insoluble fiber—have a low incidence of colon cancer.
Epidemiologists have found additional evidence for the anticancer effect of dietary fiber using the “case control” method. In this approach, researchers compare what colon-cancer patients say about their past diet with reports from noncancer subjects of similar age and background. Colon-cancer patients report having eaten significantly less fiber than do people without the disease.
Further evidence comes from animal research. Investigators have given rats various chemical carcinogens in their food or by injection while varying the animals intake of dietary fiber. In most of these studies, rats on high-fiber diets suffered fewer cancers than those on low-fiber diets. However, the results in rats have not been consistent; some studies found that the fiber had no effect, or even that it increased the cancer rate.
None of the evidence can be regarded as conclusive. The epidemiological evidence, for example, is subject to many possible interpretations. When comparing two population groups, such as Africans and Americans, there are always many differences between the groups, not only in diet but in several other aspects of life. So it’s difficult to be certain that fiber consumption is the key variable. And people who eat more fiber tend to eat less of other things, particularly fat. High-fat and high-calorie diets have been shown by many researchers to increase the risk of several kinds of cancer, including colon cancer.
There is another difficulty in interpreting the epidemiological data. What appears to be a general effect of ingested fiber may turn out to be the effect of other components in certain foods. The cruciferous vegetables, for instance—broccoli, cabbage, kale, and brussels sprouts—have been shown in some studies to have a possible protective effect against colon cancer.
Case-control studies have problems, too. It is difficult for researchers to control for all possible variables that might have caused some patients to develop colon cancer while others remained free of the disease. In addition, people may not remember accurately what types of foods they ate years ago. (Then again, there is no reason to think that colon-cancer patients should differ from other people in their recall abilities.)
The animal experiments with rats are again suggestive rather than conclusive. As noted earlier, results have been mixed. And one can’t always extrapolate reliably from the physiological reaction of one species to that of another. Moreover, even in the positive studies, not all kinds of fiber had the same protective effect, and the rats did not respond in the same way with all carcinogens.
Conflicting patterns in some earlier studies heighten the uncertainty. A few studies have reported that eating more fiber can be associated with a greater risk of colon cancer, in both rats and humans. One such report came from Australia in 1986. The researchers applied the case-control technique to 419 colon-cancer patients and 732 control subjects. They found that the colon-cancer patients had consumed slightly more cereal fiber than the controls had. The researchers narrowed down the increased cancer risk to older women, but their study may have broader implications.
Based on the Australian study and other research, some physicians believe that cancer might result from either too little or too much fiber. Many also stress the desirability of consuming fiber from a variety of sources, rather than relying on a single source.
Several of the studies on fiber are hard to interpret. Some studies—especially the older ones—fail to distinguish between soluble and insoluble fiber; there is preliminary evidence that some soluble fiber may actually increase cancer incidence. That evidence comes exclusively from rat studies involving relatively small numbers of test animals.
The issue warrants continued investigation, but does not justify changing your diet based on the risk—if any—that soluble fiber may present. At this time, CU’s consultants recommend that total fiber consumption should include both soluble and insoluble fiber. But be sure that insoluble fiber constitutes a fair part of the mix.
As research on fiber intensifies, more uncertainties emerge. There may be differences not only between soluble and insoluble fiber, but among the various types of each. Some researchers believe that the distinction between fermentable and nonfermentable fiber will prove important.
The fiber furor is not likely to be settled for a long time, if at all. The only kind of study that could settle it would be a large-scale, prospective study on humans—something on the order of the famed Framingham (Massachusetts) study on cardiovascular disease, which has spanned decades and cost millions of dollars. No such study on fiber appears likely to be undertaken.
Cautious Optimism About the Benefits of Fiber
After weighing the evidence on both sides of the controversy, scientists who are keeping close tabs on fiber research tend to be cautiously optimistic that appropriate fiber intake may indeed help to lessen people’s chances of developing colon cancer. “There is something there,” David Kritchevsky, associate director of Philadelphia’s Wistar Institute and a prominent fiber researcher, told CU. The evidence, he said, favors the conclusion that insoluble dietary fiber offers some protection against colon cancer, though “it is much too early to be jumping to conclusions.” He thinks the National Cancer Institute is pushing fiber too heavily, and he warns that some kinds of insoluble fiber actually increase the incidence of cancer in rats. In feeding rats increasingly large amounts of purified wood cellulose, he has observed an increasingly high incidence of cancer. He believes that mixtures of fiber, such as may be found in a varied diet, may offer the best protection.
Other researchers voice similar ideas. Peter Van Soest, professor of animal nutrition at Cornell University, stressed that both purified wood cellulose, and other finely ground types of fiber added to certain processed foods may do no good at all. Such highly refined fibers are broken into tiny particles that do not behave in the intestine the way the larger particles of unrefined fibers do. Coarsely ground bran, for instance, is an effective laxative, but finely ground bran can cause constipation. If you choose to sprinkle a bran supplement on top of your cereal or other foods, then it may be preferable to use one that’s fairly coarse, not one that’s ground to a powder.
Pending further research, we suggest that people increase consumption of foods containing fiber—both soluble and insoluble—rather than take fiber supplements. Eating more high-fiber foods is also likely to mean eating less fat—another beneficial measure.
How Much Fiber Should You Eat?
The National Cancer Institute recommends a daily fiber intake of between 20 and 35 grams, depending on body weight. That’s roughly double the amount of fiber the average American now consumes. CU’s medical consultants think that’s an appropriate range, but in the light of present knowledge would aim for the lower end of that range. The risk of side effects increases as you approach the upper end. Fiber consumption should not exceed 35 grams.
Rapidly adding fiber to the diet may have some side effects. A sudden, large increase in fiber intake can lead to bloating, flatulence, cramps, and diarrhea. You can avoid or minimize those side effects by adding fiber to the diet gradually.
Intestinal blockage can be caused by excessive fiber ingestion. In addition, heavy fiber intake may interfere with the absorption of minerals such as iron, copper, calcium, zinc, and magnesium. However, most researchers seem to agree that as long as people do not consume excessive levels of dietary fiber, and as long as they consume a balanced diet, they need not worry about fiber causing mineral imbalance or intestinal blockage.
Kellogg would have you get your fiber from All-Bran. The California Prune Board pushes its favorite product as “the high-fiber fruit,” saying in ads that prunes have nine grams of fiber per six-prune serving—more fiber than in two servings of many bran cereals. But comparing sources of soluble and insoluble fiber is like comparing apples and oranges (or, more nearly, apples and bread). More neutral authorities believe that relying on just one fiber source might be hazardous, and recommend that you get your fiber from a variety of foods. We agree. As long as you don’t overdo it, increasing your fiber intake probably can’t hurt, and it may help.
Last Updated on