Low-Fat, High-Fiber Diet Has No Effect On Recurrent Colon Polyps, Multicenter Study Finds

BUFFALO, N.Y. -- Adopting a diet low in fat and high in fruits and vegetables may have many health benefits, but lowering the risk of developing recurrent colon polyps, precursors of colon cancer, does not appear to be one of them.

Investigators from the University at Buffalo and the seven other centers involved in the five-year national Polyp Prevention Trial report in the April 20 issue of The New England Journal of Medicine that there was no difference in the recurrence of polyps between the diet intervention group and a control group that didn't change diets. The study was sponsored by the National Cancer Institute.

Peter Lance, M.D., UB associate professor of medicine and physiology, and principal investigator on UB's portion of the trial, said he was disappointed but not altogether surprised by the negative results.

"The study is consistent with the findings of previous smaller trials and of another large trial (Wheat Bran Fiber Study) published in the same issue. Clearly, dietary change does not make you less likely to develop new polyps after you've had all existing polyps removed.

"What the study does not address," he noted, "and was not designed to address, is what the environment of the colon should be to reduce the likelihood that small, innocent polyps will grow to become cancers."

Fifty percent or more of people over the age of 55 will develop at least one polyp of the colon during their lifetime, but only a very small number of these polyps progress to cancer, Lance said. "It remains to be seen whether, or at what stage, altering the diet keeps people from developing colon cancer," he said. "We followed these participants only for four years, and cancer develops over a number of decades. Perhaps intervening earlier in life would result in a different outcome."

Despite the negative results on polyp recurrence, Lance pointed out one promising finding of the study: People can change their diets for the better and can stick with it.

The PPT trial involved 1,905 people, all of whom had had benign colon polyps removed within the previous six months. UB contributed 262 participants, or about 12.5 percent of the total.

Although few polyps -- abnormal growths of the colon lining -- progress to cancer, cancer develops only when polyps are present. The PPT trial was designed to determine if eating a diet composed of 20 percent or less of total calories from fat, at least 18 grams of dietary fiber for every 1,000 calories consumed, and lots of fruits and vegetables could prevent polyps from growing back after they were removed.

Participants were assigned randomly to one of two groups: an intervention group, which received intensive counseling on adopting the new diet; and a control group, which received a standard brochure on healthy eating. All underwent colonoscopy at one and four years after randomization, and all completed food-frequency questionnaires throughout the study.

Based on these questionnaires, the intervention was remarkably successful in changing eating habits: Participants in that group cut their fat intake by about one-third, increased their fiber consumption by nearly 75 percent, and ate about two-thirds more fruits and vegetables than before the study. In contrast, participants in the control group made only small changes in these three diet components.

However, the percentage of participants who developed at least one recurrent polyp during the study was nearly identical in both groups: 39.7 percent (intervention) and 39.5 percent (controls). In addition, there was no difference between the groups in the mean number of recurrent polyps per person, nor in their size or degree of progression.

The researchers noted that despite these findings, the idea that a healthy diet may lower the risk of colon cancer should not be discarded entirely. Several factors, including study length and potential reporting errors, could have influenced this study's findings, Lance said. In addition, such a diet in known to have a favorable impact on cardiovascular disease and other chronic conditions.

In addition to UB, the seven clinical sites involved in the study were Veterans Administration Medical Center, Hines, Ill.; Kaiser Foundation Research Institute, Oakland, Calif.; Memorial Sloan-Kettering Cancer Center, New York City; University of Pittsburgh; University of Utah, Salt Lake City; Wake Forest School of Medicine, Winston-Salem, N.C., and Walter Reed Army Medical Center, Washington, D.C.