CLINICAL-TRIAL OF ANTIOXIDANT VITAMINS TO PREVENT COLORECTAL ADENOMA

Background. People who consume a diet high in vegetables and fruits ha ve a lower risk of cancer of the large bowel. Antioxidant vitamins, wh ich are present in vegetables and fruits, have been associated with a diminished risk of cancers at various anatomical sites. We conducted a randomized, controlled clinical trial to test the efficacy of beta ca rotene and vitamins C and E in preventing colorectal adenoma, a precur sor of invasive cancer. Methods. We randomly assigned 864 patients, us ing a two-by-two factorial design, to four treatment groups, which rec eived placebo, beta carotene (25 mg daily), vitamin C (1 g daily) and vitamin E (400 mg daily), or beta carotene plus vitamins C and E. In o rder to identify new adenomas, we performed complete colonoscopic exam inations in the patients one year and four years after they entered th e study. The primary end points for analyses were new adenomas identif ied after the first of these two follow-up examinations. Results. Pati ents adhered well to the prescribed regimen, and 751 completed the fou r-year clinical trial. There was no evidence that either beta carotene or vitamins C and E reduced the incidence of adenomas; the relative r isk for beta carotene was 1.01 (95 percent confidence interval, 0.85 t o 1.20); for vitamins C and E, it was 1.08 (95 percent confidence inte rval, 0.91 to 1.29). Neither treatment appeared to be effective in any subgroup of patients or in the prevention of any subtype of polyp def ined by size or location. Conclusions. The lack of efficacy of these v itamins argues against the use of supplemental beta carotene and vitam ins C and E to prevent colorectal cancer. Although our data do not pro ve definitively that these antioxidants have no anticancer effect, oth er dietary factors may make more important contributions to the reduct ion in the risk of cancer associated with a diet high in vegetables an d fruits.