Trials Show No Effect of Low-Fat, High-Fiber, and High-Fruit and -Vegetable Diets on the Growth of New Colorectal Polyps in People with a History of Precancerous Polyps
Two studies of nutritional interventions in people at high risk of colorectal cancer yielded no evidence that diet affects the growth of precancerous colorectal polyps (adenomatous polyps or adenomas) in people who have already had a polyp removed. Colorectal cancer is the second leading cause of cancer death in the United States and polyps are the precursors to most colorectal cancers. The results were published today in the New England Journal of Medicine.
The studies the Polyp Prevention Trial and the Wheat Bran Fiber Study looked at the effect of diet on the growth of new colorectal polyps because people with a prior polyp have as much as a 50 percent chance of developing more polyps within three years of their diagnosis. Between 5 percent and 10 percent of precancerous colorectal polyps will become cancer if not removed. Removal of polyps reduces colorectal cancer risk.
"Polyps are a point where we can intervene to prevent colorectal cancer," explained Arthur Schatzkin, Ph.D., Dr.P.H., chief of the National Cancer Institute's (NCI's) Nutritional Epidemiology Branch and a lead investigator in the Polyp Prevention Trial. "These trials were done to test whether dietary change could prevent these people from getting more polyps and thereby reduce their risk of colorectal cancer. We were surprised and disappointed to find that the dietary changes made in these studies did not decrease the number of new polyps the participants developed," he said.
"Although we still have a ways to go to prove that dietary change can prevent colorectal cancer, there is a great deal of evidence that adopting a diet low in animal fat, high in whole grains, and rich in vegetables and fruit can improve one's overall health and reduce the risk of chronic disease," said Schatzkin.
In the Polyp Prevention Trial, 2,079 men and women who had one or more polyps removed within the prior six months were randomized (divided by chance) to have either intensive counseling to adopt a low-fat, high-fiber, fruit- and vegetable-enriched eating plan or to be given a standard brochure on healthy eating. The eating plan was 20 percent of calories from fat and 18 grams of fiber and 3.5 servings of fruits and vegetables for each 1,000 calories eaten (most adults eat between 1,800-2,500 calories per day). The participants were in the trial for four years.
"We need to conduct further research to understand the complex interactions between the dietary and genetic factors that lead to colorectal cancer," explained Elaine Lanza, Ph.D., a lead investigator on the Polyp Prevention Trial and a researcher in NCI's Cancer Prevention Studies Branch. "Because only 5 percent to 10 percent of polyps progress to colorectal cancer, we need to understand the molecular differences that define the polyps which do progress, and to study the effect of diet in individuals with these particular types of polyps."
Polyp Prevention Trial researchers will continue to follow the participants in the trial to see if the dietary changes they made during the trial influence their likelihood of developing polyps or colorectal cancer in the future. In addition, they are using tissue from the polyps that were removed during the trial to find out whether diet affects the molecular genetic characteristics of the polyps.
David S. Alberts, M.D., director of cancer prevention at the Arizona Cancer Center, Tucson, Ariz., and lead investigator in the Wheat Bran Fiber Study noted, "Although neither of these studies gave us a strategy to prevent polyp recurrence, we have learned a tremendous amount about the clinical, pathological, and genetic factors associated with polyp growth and recurrence."
In the Wheat Bran Fiber Study, 1,429 men and women from the greater Phoenix area who had one or more polyps removed within the prior three months were randomized to a high wheat bran fiber cereal supplement (13.5 grams of fiber in 2/3 cup of cereal per day) or low wheat bran fiber cereal supplement (2 grams of fiber in 2/3 cup cereal per day). The participants were on the supplements for at least three years.
"The results of the high-fiber intervention are compatible with recent observational data from epidemiologic studies that suggest that increasing dietary fiber won't decrease colorectal cancer risk," said Alberts. "While dietary changes involving fiber may not benefit colorectal cancer risk, dietary fiber may help with chronic diseases of Western culture that involve the colon and cardiovascular system."
Although the interventions did not reduce the number of colorectal polyps that the participants developed, the scientists offered several possible reasons why these study designs did not show an effect of diet on polyp recurrences, including:
- Development of colorectal cancer takes decades, and an intervention of three to four years may not be long enough to make a difference. Continuing follow-up of these patients may lead to further understanding of the long-term impact of dietary interventions.
- Participants in these studies all had at least one polyp previously removed. Nutritional factors may influence critical molecular, cellular, or tissue-level events in colorectal cancer formation well before polyps are formed.
- In these studies, the recurrent polyps the new polyps that develop after the first ones are removed tended to be small. Dietary changes might affect only the growth of small polyps into large polyps or large polyps into invasive cancer.
The investigators emphasized that a diet high in fiber, fruits, and vegetables and low in fat can improve overall health and reduce the risk of chronic diseases, including heart disease, hypertension, diabetes, and others. The studies published today looked at a specific scientific question namely, does dietary change affect recurrence of colorectal polyps in a group of people who already had polyps.
The Polyp Prevention Trial was funded by NCI and took place at eight clinical centers across the United States: State University of New York at Buffalo, Buffalo, N.Y.; Edward Hines Jr. Hospital, Veterans Administration Medical Center, Hines, Ill.; Kaiser Foundation Research Institute, Oakland, Calif.; Memorial Sloan-Kettering Cancer Center, New York; University of Pittsburgh, Pittsburgh, Pa.; University of Utah, Salt Lake City; Wake Forest University Baptist Medical Center, Winston-Salem, N.C.; and Walter Reed Army Medical Center, Washington, D.C.
About two-thirds of the participants in the Polyp Prevention Trial were male and about 10 percent were minorities. The average age when they joined the trial was 62.
The Wheat Bran Fiber Study was funded by NCI, headed by the Arizona Cancer Center, and carried out by the Phoenix Colon Cancer Prevention Physician Network. About two-thirds of the participants were male, and about 3 percent were minorities. The average age when they joined the study was 66.
The investigators are grateful to the study participants for their contribution to these studies, which required participants to be consistent in the changes in their diets over a long period of time.
NCI is sponsoring a number of prevention trials in which drugs or nutritional supplements are being tested to reduce risk of developing polyps and/or colorectal cancer. Agents under study include sulindac and celecoxib (anti-inflammatory agents that inhibit an enzyme known as cyclo-oxygenase), and the nutrients folic acid and calcium.
Lanza noted, "Until we have the results of these new trials, the only proven way to prevent colorectal cancer is routine screenings for the detection and removal of polyps."
To learn more about participating in prevention trials or about colorectal cancer, contact NCI's Cancer Information Service at 1-800-4-CANCER or visit NCI's Web site at http://cancer.gov.
Schatzkin A, Lanza E, Corle D, et al. Lack of effect of a low-fat, high-fiber diet on the recurrence of colorectal adenomas. N Engl J Med 2000:342:1149-55.
Alberts DS, Martinez ME, Roe DJ, et al. Lack of effect of a high-fiber cereal supplement on the recurrence of colorectal adenomas. N Engl J Med 2000:342:1156-62.