The Polyp Prevention Trial and the Wheat Bran Fiber Study: Questions and Answers
- What is a colorectal polyp?
A colorectal polyp is a growth on the inner wall of the colon or rectum. Precancerous polyps (adenomatous polyps or adenomas) are fairly common in people over age 50 it is estimated that about 40 percent of people over age 50 have them. Because most colorectal cancers develop in polyps, detecting and removing these growths is a way to prevent colorectal cancer. About 5 percent to 10 percent of polyps will become cancer if they are not removed, although physicians cannot yet identify which polyps are likely to lead to cancer. People who have had a polyp have an increased risk perhaps as much as 50 percent of developing more polyps within three years of their first diagnosis.
- What is the Polyp Prevention Trial?
The Polyp Prevention Trial was a clinical trial (a research study conducted with volunteers) to determine the effect of a low-fat (20 percent of calories from fat), high-fiber (18 grams per 1,000 calories), high fruit/vegetable (3.5 servings per 1,000 calories) eating plan on the recurrence of precancerous polyps in the colon and rectum. A group of 2,079 men and women were randomized (divided by chance); half had intensive counseling to adopt a low-fat, high-fiber, fruit- and vegetable-enriched eating plan and half were given a standard brochure on healthy eating. Every participant had been diagnosed with a polyp within the previous six months, which was removed at colonoscopy. The participants were on the trial for four years. The trial was headed by the National Cancer Institute (NCI) and carried out at eight clinical sites known as the Polyp Prevention Trial Study Group.
- What were the results of the Polyp Prevention Trial?
The Polyp Prevention Trial provided no evidence that adopting a low-fat, high-fiber, fruit- and vegetable-enriched eating plan reduces the recurrence of colorectal polyps. Polyp recurrence rates were about the same in the two study groups.
- What is the Wheat Bran Fiber Study?
The Wheat Bran Fiber Study was a clinical trial to assess the role of a wheat bran fiber supplement in the prevention of colorectal polyp recurrence. A group of 1,429 men and women who had had one or more polyps removed at colonoscopy within the previous three months were randomized to a high wheat bran fiber cereal supplement (13.5 grams of fiber in 2/3 cup 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 study was funded by NCI, headed by the Arizona Cancer Center, Tucson, Ariz., and carried out by the Phoenix Colon Cancer Prevention Physician Network.
- What were the results of the Wheat Bran Fiber Study?
The Wheat Bran Fiber Study provided no evidence that adding a wheat bran fiber cereal supplement to the diet reduces the recurrence of colorectal polyps.
- What do these results mean in terms of reducing polyp recurrence via dietary modification?
The results provided no evidence that the particular dietary interventions employed (i.e., a low-fat, high-fiber, high-fruit and -vegetable eating plan or a high-fiber cereal supplement) in the particular population studied (individuals who had had one or more polyps removed at colonoscopy) were effective in preventing the recurrence of polyps. However, overall evidence suggests that a low-fat, high-fruit and -vegetable, high-fiber diet has benefit in reducing the risk of many chronic diseases heart disease, hypertension, obesity, diabetes, and others. This trial specifically looked at the effect of diet on the growth of new colorectal polyps in people who had already had a polyp removed. A healthy diet does not replace the need for people with a history of polyps to have regular checkups.
- Why didn't these trials show a protective effect of dietary changes or fiber supplements on colorectal polyps?
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.
- Why look at diet to prevent polyps?
Dietary factors are believed to influence colorectal cancer risk, and polyps are considered a mid-way point to colorectal cancer. If diet was found to change the risk of polyp recurrence, that would be strong evidence that diet could reduce the risk of colorectal cancer. Scientists would also have evidence of the point at which diet influences colorectal cancer risk.
- Is there evidence that diet affects colorectal cancer risk?
Yes. Colorectal cancer rates vary widely from country to country; studies of people who emigrate show that as they adopt a diet of the new country, their colorectal cancer risk becomes more like the country to which they moved. This shows that while genetic susceptibility (heredity) may play a role in colorectal cancer, outside (environmental) factors also influence risk. Studies in laboratory animals have shown that fat increases and fiber decreases risk of colorectal tumors. Many epidemiologic studies looking at what people eat (observational studies) have suggested that red meat and dietary fat increase colorectal cancer risk while vegetables, dietary fiber, and some nutrients lower the risk. For more information, see the PDQ® Colorectal Cancer Screening summary.
- Are other large trials looking at the role of diet in cancer prevention?
The Women's Health Initiative, an ongoing 15-year study supported by the National Institutes of Health and administered by the National Heart, Lung, and Blood Institute involves 68,000 women in a clinical trial examining the effect of a diet low in fat and high in fruit, vegetables, and grains in preventing breast and colorectal cancers and heart disease. The Women's Health Initiative is also testing the ability of calcium and vitamin D supplements to prevent fractures and reduce the risk of colorectal cancer. More information on the study is available at http://www.nhlbi.nih.gov/whi on the Internet.
- What other cancer prevention trials are under way for colorectal cancer?
The NCI is sponsoring a number of prevention trials in which drugs or nutritional supplements are being tested to reduce the 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.
- What are the next steps?
Polyp Prevention Trial researchers will continue to follow the participants in the trial to see if the dietary changes they made influence their likelihood of getting more polyps or colorectal cancer in the future. In addition, they are using the material from the polyps that were removed to find out whether the diet affected the molecular genetic characteristics of the polyps.
The Wheat Bran Fiber Study researchers will also continue to follow the participants in their trial to see if they develop any additional polyps or colorectal cancer in the future. The researchers will follow study participants for lifestyle, genetic, and clinical factors that influence polyp recurrence.
- Who is at risk for colorectal cancer?
Colorectal cancer results from complex interactions between a person's genetic make-up and outside factors. Science has shown that these interactions take decades to develop. Some of the factors that put people at risk include:
- age (most cases occur in people over age 50)
- personal history of polyps, colorectal cancer, or inflammatory bowel disease
- family history of polyps or colorectal cancer (in close relatives)
- smoking, and
- sedentary lifestyle.
- What tests are used to screen people for colorectal cancer? Do insurance companies pay for colorectal cancer screening?
People who are at risk for colorectal cancer should ask their doctor when to begin screening, what tests to have, and how often to schedule appointments. Doctors may suggest one or more of the tests listed below as a part of regular checkups.
A fecal occult blood test (FOBT) is a test for hidden blood in the stool. This test has been proven to reduce the death rate of colorectal cancer.
A sigmoidoscopy is an examination of the rectum and the lower part of the colon with a lighted instrument.
A colonoscopy is an examination of the rectum and entire colon with a lighted instrument.
A double contrast barium enema is a series of X-rays of the colon and rectum. The X-rays are taken after the patient is given an enema with a white, chalky solution that contains barium to outline the colon and rectum on the X-rays.
A digital rectal exam or DRE is a test in which the doctor inserts a lubricated, gloved finger into the rectum to feel for abnormal areas.
People should check with their health insurance provider to determine their colorectal cancer screening benefits. People who are age 50 or older and are covered by Medicare are eligible for colorectal cancer screening benefits. Additional information is available on the Medicare homepage at http://www.medicare.gov on the Internet.