Nutritional factors have long been thought to play important roles in the development of degenerative diseases. In particular, increasing dietary fiber consumption has been proposed as a method for preventing colon cancer. Unfortunately, definitively determining the role dietary factors play in complex diseases is extremely challenging. All of the scientific evidence concerning dietary fiber and colon cancer in humans is of necessity indirect in one way or another, and the different types of indirect evidence have produced conflicting results.
Randomized Trials
The most definitive medical evidence comes from randomized trials. An ideal randomized trial for examining the role of dietary fiber in human colon cancer would randomly assign people to follow either a high-fiber or a low-fiber diet. The people in both diet groups would then be followed for a period of time sufficient to allow a significant number of cancer cases to develop, and the rates of cancer in the two groups could be compared. The random assignment would ensure that any difference between the groups was the result of the diets rather than other factors, and the significant number of cancer cases would allow results of the diets to be compared with great precision. Unfortunately, such a study would be utterly impractical due to the real-world difficulty of getting people to make long-term dietary changes and due to the necessity of following a large group of people for a long period of time in order to see enough cases of colon cancer, which develops relatively slowly in humans.
Animal Studies
One way to get around the practical problems of randomized trials in humans is to use animals instead. It is fairly simple to completely control the diet of an experimental animal, and colon cancer develops much more quickly in some animals whose lifespans are shorter than humans. A review article published in the 1999 "Annual Review of Nutrition" concluded that most studies involving animals, usually rats, have found that increased dietary fiber does in fact prevent colon cancer. The primary limitations of these studies are the significant known differences in rat and human nutrition. So what prevents colon cancer in rats may not necessarily prevent it in humans.
Modified Randomized Trials in Humans
Another practical method for examining the effect of dietary fiber on colon cancer is to study humans while using a simpler dietary change and a less direct measure of success. A typical example of such a study was published in the April 20, 2000 issue of the "New England Journal of Medicine." Conducted by David S. Alberts, MD, and colleagues, this study took 1303 patients who had already been successfully treated for colon cancer and randomly assigned them to consume either a high-fiber cereal supplement or a similar supplement that was low in fiber. After about three years of follow-up, the rates of colon cancer recurrence in the two groups were compared. Consistent with most other studies of this kind, no significant difference between the two groups was found. These kinds of studies are practical because it's relatively easy to get people to consume a fiber supplement regularly and because colon cancer is much more likely to recur than it is to occur for the first time. Unfortunately, a dietary supplement may not accurately reflect the type and source of fiber that people actually include in their diets, and the effects of fiber on recurrence may be different from its effects on initial occurrence.
Population-Based Studies
Another practical approach continues to look at humans, actual high-fiber diets and the initial occurrence of colon cancer, but dispenses with randomization. Population-based studies typically recruit a large group of people who have never had colon cancer and determine their fiber consumption. After enough time has elapsed, the researchers determine which people have developed cancer and see if there is any association between fiber consumption and the risk of cancer. Because it is relatively simple to determine someone's fiber consumption, it is practical to follow a large group for long enough to look at the initial occurrence of cancer. Because there is no randomization, however, it can be difficult to determine if it is really the amount of fiber in the diet that is affecting the rate of colon cancer. For example, people who eat more fiber may happen to eat less fat as well, and it may actually be the decreased fat consumption that decreases the risk of cancer.
These sorts of potentially confounding factors need to be carefully assessed in population-based studies so that their effects can be statistically controlled. An article by Y. Park and colleagues published in the December 14, 2005 issue of the "Journal of the American Medical Association" pooled the results of several large, population-based studies on fiber and colon cancer and concluded that while dietary fiber is associated with lower cancer rates, the effect disappears when controlling for other dietary factors, such as fruit and vegetable consumption.
Conclusions
Because there is no definitive evidence on the role of dietary fiber in colon cancer and because the indirect evidence currently available is inconsistent, at present it is impossible to say with any certainly whether or not increasing dietary fiber consumption will decrease the risk developing cancer. However, the results of the population-based studies do suggest that looking at a single dietary factor, such as fiber consumption, in isolation may be too simplistic. For example, a population-based study by H. Wu and colleagues published in the February 2009 issue of the "Journal of Nutrition" found that consumption of fruits and green vegetables was associated with a decreased risk of colon cancer, even after controlling for other dietary factors. Many fruits and vegetables are high in fiber, but they contain other compounds that are probably cancer-protective as well. At the moment, the best advice for someone interested in decreasing his risk of colon cancer would be to increase the fiber in his diet by increasing his consumption of high-fiber fruits and vegetables.
References
- "Annual Review of Nutrition"; "Dietary Factors in Human Colorectal Cancers"; M Lipkin, et al.; 1999
- "New England Journal of Medicine"; "Lack of Effect of a High-Fiber Cereal Supplement on the Recurrence of Colorectal Adenomas"; DS Alberts, et al,; April 20, 2000
- "Journal of the American Medical Association"; Dietary Fiber Intake and Risk of Colorectal Cancer "; Y Park, et al.; Dec. 14, 2005
- Journal of Nutrition: Fruit and Vegetable Intakes Are Associated with Lower Risk of Colorectal Adenomas
