Fiber: Fact & Fiction - dietary fiber - Brief Article

Vegetarian Times, July, 2000 by Abigail Chipley

If you believe recent news stories debunking high-fiber diets as a way to stave off colon cancer, you might as well toss those oat bran muffins, fruits and vegetables into the compost heap.

Two studies involving thousands of people and published in April's New England Journal of Medicine concluded that neither consuming a high-fiber, low-fat diet nor eating extra wheat-bran fiber made any difference in preventing colon cancer, the second-leading cause of cancer-related deaths after lung cancer. And though most articles written on the subject seem to accept these findings as the gospel truth, the case against fiber is hardly airtight.

In both studies, researchers examined the effects of diet on men and women age 35 and older who had previously had at least one polyp, a tiny noncancerous growth, removed from their colons. (Since the disease often starts out as a polyp, these people face an increased risk for developing colon cancer.) In the first, a four-year study led by Arthur Schatzkin, M.D., chief of nutritional epidemiology at the National Cancer Institute (NCI), 2,079 participants were randomly assigned to eat either a low-fat, high-fiber diet with lots of fruits and vegetables or to follow their regular diets. The second, a three-year trial done at the University of Arizona and involving more than 1,400 people, focused on fiber alone. Researchers divided participants into two groups: One ate high-fiber wheat-bran cereal or bars and the other ate low-fiber cereals or bars.

Both research teams were seeking evidence to prove the accepted hypothesis that fiber protects against colon cancer by moving food through the bowel faster, thereby decreasing the time the body is exposed to cancer-causing substances in meat and other foods. But neither study bore this theory out. In both trials, almost 40 percent of the participants who changed their diets had recurrent polyps--a percentage only slightly smaller than the control groups'. Says Schatzkin, "We were surprised and disappointed. We had high expectations that we would see a positive result with a low-fat, high-fiber diet, and we didn't."

Not all the experts were surprised, however. Dean Ornish, M.D., a pioneer in reversing heart disease through diet and president of the Preventive Medicine Research Institute in Sausalito, Calif., explains the inherent weakness of studies that rely on participants to monitor their own behavior. "When patients know that you want them to follow a certain diet, to please you they'll say they are doing it, even if they aren't." In fact, key health indicators measured by the researchers seem to support the notion that the participants might have misrepresented the truth. In the NCI study, participants' fat intake was supposedly reduced from 36 to 24 percent of calories, partly by decreasing red meat and increasing fruit and vegetable intake. Yet over four years their cholesterol levels barely budged and they lost, on average, just one pound. "If they were really adhering to this diet, I think you'd see more of a change," says Ornish. "We know from a number of studies that if you reduce your intake of fat significantly, you get a big drop in both cholesterol and weight."

The findings of the University of Arizona study are also suspect. Though slightly better designed than the NCI study (the participants were provided with the cereals and had to bring back empty boxes), the Arizona study was still flawed. For example, the group that ate high-fiber breakfast cereals or bars contained nearly twice as many smokers, and members of this group also had higher fat and alcohol intakes than those in the control group.

Ornish says it is a mistake to focus on only one element of the diet--like fiber--and assume that it alone affords protection from disease. "It's really the totality of the diet that's important," he says. "Fiber isn't the only thing here. Meat and dairy have been linked with colon cancer as well."

Of the hundreds of articles written about these studies, most glossed over the quality of the evidence presented, emphasizing instead the fact that fiber has been proven to lower your risk for developing high blood pressure, adult-onset diabetes and heart disease. Those are certainly good reasons to load up on veggies and grains, but the issue of fiber's role in preventing colon cancer is still very much open to debate. Even Schatzkin is hedging his bets. "We plan to follow the participants for five more years. And who knows? We may see different results at year eight or nine," he says.