Introduction

Health experts say that a healthful diet can reduce the risk of colorectal cancer. What they've yet to agree on is how individual dietary components--fiber, fat, calcium--can affect that risk. One nutrient that is in particular flux is fiber, as two studies released earlier this year cast some doubt on the possible cancer-protective benefits of a high-fiber diet. To add to the confusion, a study in a recent issue of the journal Lancet suggests that fiber supplements may actually promote colon growths in some people.

What role for fiber in colon cancer prevention?

For this latest analysis, European researchers selected 552 people with a history of colorectal adenomas. (Adenomas, or growths, on the wall of the colon are a risk factor for cancer). Participants were divided into three groups. One group received 2 grams of calcium supplement; one consumed 3.5 grams of soluble fiber supplement; and the people in the third group received an inactive (placebo) supplement. Doctors initially examined the colons of participants by means of colonoscopy, and removed most visible adenomas. The participants were examined again at the end of the three-year study.

The scientists report that about 16% of those who received supplemental calcium and 29% of those who received fiber supplements developed further adenomas over the three years of the study. In contrast, 20% of those who received the placebo supplements had adenomas removed at the end of the study. With these results, the scientists concluded that calcium supplements gave people some slight protection from the growths that often precipitate colon cancer, but that fiber supplements actually increased the participants' risk of developing these pre-cancerous growths.

Fiber supplements don't make a high-fiber diet

Did the fiber supplement used in this study actually cause more growths? Several flaws in the study's design make this difficult to judge. But the results of this study do highlight the shortcomings of relying on dietary supplements to try to make a diet "healthier." Study participants were not required to make any changes to their diets or lifestyles other than to consume the supplements. The 3.5 grams of soluble fiber tested in this study is equal to about one rounded teaspoon of a fiber supplement like Metamucil.
That's enough to help keep a person "regular," but not enough to substantially increase the fiber content of a person's diet.

In fact, the authors of this study say that their findings should not keep people from choosing a diet that includes a variety of fresh fruits and vegetables and whole grain foods. These foods contain nutrients and substances other than fiber that are needed for good health. The American Institute for Cancer Research points out that vegetables, for instance, are not only a good source of fiber, but also provide carotenoids, vitamin C, folic acid, and many kinds of phytochemicals, all of which may help reduce the risk of colon cancer. In other words, this study has not altered the definition of a healthful diet. It has, though, illustrated the difficulty of assigning disease-fighting properties to individual food components, in this case, calcium and fiber.