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As Clinical Studies Mature, Dietary Factors that Prevent Colon Cancer May Be Defined

As Clinical Studies Mature, Dietary Factors that Prevent Colon Cancer May Be Defined

With the maturation of several clinical trials in the late 1990s, oncology researchers are on the verge of determining which types of nutritional interventions will be effective in the primary prevention of colon cancer, David S. Alberts, md, said at the National Conference on Colorectal Cancer, sponsored by the American Cancer Society.

The use of dietary elements to prevent colon cancer has had a spotty record in recent clinical trials. No intervention effect from the antioxidants beta-carotene and vitamins C and E was demonstrated at colonoscopy after 1 year in a study of 864 patients reported by Dr. Robert Greenberg of Dartmouth University in 1994.

However, a diet high in unrefined wheat bran fiber and low in fat prevented the recurrence of polyps larger than 10 mm over a period of 4 years in a randomized blinded study of 424 patients, presented by Dr. Finlay Macrae of the Royal Melbourne Hospital in May 1995.

"This is the first study showing a positive association between nutritional interventions and sporadic colon polyp prevention done with a randomized blinded design that we can build upon," said Dr. Alberts, director, Prevention & Control, Arizona Cancer Center, Tucson.

Dr. Alberts observed that four major clinical trials of nutritional interventions will provide clues concerning the importance of lowering the amount of dietary fat and increasing the consumption of fruits and vegetables, wheat bran, calcium carbonate, and selenium.

NCI Intervention Study

The Nutritional Intervention Study, under the direction of Dr. Arthur Schatzkin, of Memorial Sloan-Kettering Cancer Center, is being conducted through the National Cancer Institute at eight separate sites. The study includes 2,000 post-polypectomy patients who have been randomized to follow either a diet low in fat (less than 20% of daily calories from fat) and high in fruits and vegetables (five to eight servings a day) or the standard American diet.

The average patient in the intervention arm of the study is consuming approximately 25% of daily calories from fat and six servings of fruits and vegetables each day, Dr. Moshe Shike said at the conference. This contrasts with the usual American diet, which consists of 35% of calories from fat and only two to three daily servings of fruits and vegetables.

The objective of the Nutritional Intervention Study is to examine the interaction of fruits and vegetables and a low-fat diet, not the effect of a single dietary component, on the recurrence of colorectal polyps at colonoscopy. The study has accrued all of its subjects and will conclude in 1998 or 1999, Dr. Shike said.

Another dietary intervention study, from the University of Arizona, is following 1,429 patients from the Phoenix metropolitan area who had an initial index polyp resected. The patients are being randomized to receive wheat bran in the form of cereal or breakfast snack bars, either 2 or 13.5 g/d, for up to 5 years. The end point of the study will be the results of colonoscopy between 3 and 4 years.

Effects of Calcium and Selenium

A multicenter study of the effect on polyp recurrence of consuming 1,200 mg/d of calcium carbonate for 3 years has been completed and is under analysis. The study is being coordinated by Dr. John Baron of Dartmouth University, and includes 900 patients who have undergone resection for adenomatous polyps.

A recently completed multicenter study of selenium in skin cancer patients also may have a bearing on colon cancer, Dr. Alberts said. Under the direction of Dr. Larry Clark of the University of Arizona, the study evaluated the use of 200 or 400 mcg of selenium in 1,700 individuals with skin cancer that had been resected. The preliminary analysis of the results of the intervention has been completed, and a paper has been submitted for publication.

 
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