Dietary changes don't prevent recurrent polyps, eight-center study shows

April 19, 2000

The Polyp Prevention Trial, one of the largest studies aimed at preventing colon cancer by dietary change, came to an unexpected conclusion:

"The Polyp Prevention Trial provided no evidence that adopting a low-fat, high-fiber fruit- and vegetable-enriched eating plan reduces the incidence of colorectal cancer," according to a report in today's (April 20) issue of the New England Journal of Medicine.

The study, which involved Wake Forest University Baptist Medical Center and seven other centers, tested the effect of diet on recurrence of intestinal polyps. Polyps are considered a precursor of cancer of the colon. The research team had hoped that the study would establish the importance of diet in preventing this cancer.

Instead, for people who already have had polyps, the research team is now recommending regular screening and colonoscopies, according to Elaine Lanza, Ph.D., of the National Cancer Institute, national co-principal investigator.

"This has led us in other directions in preventing colon cancer," said M. Robert Cooper, M.D., principal investigator at Wake Forest and professor of internal medicine (hematology/oncology).

Besides Medical Center patients, Cooper's team recruited participants from private practicing gastroenterologists in Forsyth County, Charlotte, Greensboro, Burlington and Statesville. Half the patients were assigned to the dietary group, half to a control group.

The diet included five to eight servings of fruits and vegetables a day, at least 18 grams of dietary fiber a day for every 1000 calories, and no more than 20 percent of daily calories from fat. The participants on the diet also were assigned a nutritionist for counseling and attended more than 50 hours of individual and group dietary counseling sessions.

The control group got general dietary guidelines from the National Dairy Council -- but no additional nutritional or behavioral help. The trial lasted for four years. Doctors repeated colonoscopies to look for recurring polyps at the end of the first and fourth years.

The participants seem to have followed the diet. The investigators report that the dietary group consumed 9.7 percent fewer calories from fat than the control group and increased their fiber intake by nearly 75 percent. Servings of fruits and vegetables increased by about two-thirds in the intervention group, while control group participants only had a slight increase. Consumption of red meat dropped by about 20 percent in the diet group but only 2.5 percent in the control group. Use of whole grains increased 38 percent in the diet group, but declined in the control group.

Nonetheless, polyps recurred in 39.7 percent of the diet group and 39.5 percent of the control group, a statistical tie.

"We also found no effect of the dietary intervention on either large or advanced lesions," the group reported.

The researchers analyzed a long list of possible explanations, including such considerations that "dietary adherence was less than it appeared," or "the reported changes in intake of fat, fiber and fruits and vegetables were real - but not big enough" or "we intervened too late in life."

They concluded, "The plausibility of several alternative explanations for the…findings argues against concluding definitively that dietary change is ineffective."
Other centers included Memorial Sloan-Kettering Cancer Center in New York, the State University of New York at Buffalo, the University of Pittsburgh, the University of Utah in Salt Lake City, Walter Reed Army Medical Center in Washington, the VA Medical Center in Hines, Ill., and the Kaiser Foundation Research Institute in Oakland, Calif.

Wake Forest Baptist Medical Center

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