Because science is such a dynamic process, you can never exactly tell where it is going to lead you. Conclusions that once seemed logical and fairly solid may be revised—or completely overturned—as more and better research is done on a particular topic. One example of this is the relationship between fiber and colon cancer.
Starting about 30 years ago, a high fiber intake was regularly recommended as one way to lower the risk for colon cancer. This recommendation was largely based on observations that countries with a high fiber intake tended to have rates of colon cancer lower than the rates found in countries with a low fiber intake.
But such descriptive studies don’t provide the most definitive information. While they are often good points to start a scientific journey, they only take a broad look at large groups of people. Descriptive studies generally can’t address all of the factors that might account for differences in rates of disease. Fiber intake could indeed have something to do with the differences in colon cancer rates, but those differences could also involve many other things that differ between countries, including other diet or lifestyle factors.
When studies that can take such things into account on an individual level began to look at the issue of fiber and colon cancer, the picture became much less clear. A number of case-control studies found that a high fiber intake was linked to a lower risk of colon cancer, but many did not. Given these wavering results—and because case-control studies are not an optimal way to assess food intake, relying as they do on participants’ recollections of what they ate in the past—more research using better methods was needed. In the meantime, many health professionals still regularly recommended a high fiber intake for people trying to lower their risk of colon cancer.
Not until the results of cohort studies came out did this recommendation begin to lose its backing. Because cohort studies observe a group of people over time, their findings are generally stronger than those of case-control studies, especially when it comes to something like diet and colon cancer. What most of these cohort studies found was that fiber intake had very little, if any, link with colon cancer.
Such findings were further bolstered by the results of randomized trials—types of studies that many consider the gold-standard of research. These studies took a group of people and randomly assigned individuals to one of two groups. One group was put on a high fiber diet, while the other group followed a lower fiber diet. After 3 to 4 years, the two groups were compared and no difference was found in rates of colon polyps—noncancerous growths that can turn into cancer. Of course, colon polyps are not cancer, but since it’s thought that all colon cancers start as polyps, it is strong evidence that fiber intake has no direct link with colon cancer.
In this case, the path of discovery led from widespread belief in a clear link between fiber and colon cancer to acceptance of the likelihood that there was no strong link between the two. As such, it’s an excellent example of how research can often develop. What may start as a clear connection based on findings from broad, descriptive studies can slowly unravel as more and better-quality research unveils the true nature of a relationship. However, keep in mind that a weak relationship is difficult to exclude altogether. Further studies might yet demonstrate a weak effect of fiber on colon cancer, although such a finding wouldn’t alter the conclusion that other means must be sought to prevent colon cancer.
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