23 Oct 1999--For men concerned about prostate-cancer risk, here's some good news from researchers at the Fred Hutchinson Cancer Research Center in Seattle:

• vasectomy does not increase a man's risk of prostate cancer, contrary to several earlier studies that have suggested a link; and
• dietary supplementation with zinc, vitamin C or vitamin E may significantly reduce the risk of the disease.

"We looked carefully at this large population with a high prevalence of vasectomy and didn't see an association with prostate cancer," says Stanford, a member of the Center's Public Health Sciences Division and a research professor of epidemiology at UW. "This is an important public-health question, and we believe that our results will be reassuring to physicians performing the procedure and to couples selecting vasectomy for contraception," she says. Vasectomy costs less than tubal ligation, the surgical-contraceptive counterpart for women, and is associated with fewer side effects.

While several other recent studies also have found no association between vasectomy and prostate cancer, the National Cancer Institute commissioned the Hutchinson Center study to address lingering doubt stemming from earlier studies that had suggested a link. Researchers from the University of Washington School of Public Health and Community Medicine and Seattle's Northwest Prostate Institute also collaborated on the study.

While some of the earlier studies were flawed substantially by factors ranging from selection bias to second-hand data collection, other investigations of more rigorous scientific design reported conflicting results that raised legitimate concern about a possible association between vasectomy and prostate cancer, Stanford says.

The Hutchinson Center's five-year study of 1,456 men ages 40 to 64 compared 753 men in King County who had prostate cancer to 703 who did not. In addition to whether they'd had a vasectomy, the men answered questions regarding medical history, screening for prostate cancer, and a number of lifestyle choices, including diet and the use of vitamin and mineral supplements. The research team adjusted for variables such as age, race and family history of prostate cancer. The results showed that the prevalence of vasectomy within the two groups was similar: 39.4 percent of those with prostate cancer had undergone vasectomy compared to 37.7 percent of the non-cancer group. After adjustment for confounding factors, there was no association found between vasectomy and risk of prostate cancer.

Effects of Vitamins and Supplements
The researchers also questioned the men about the frequency and duration of dietary-supplement use, specifically regarding four types of multivitamins (with and without minerals) and six individual nutritional supplements: vitamins A, C and E, and calcium, iron and zinc.

Analysis of the supplement data indicates that fewer than one-third of the men in both groups took daily multivitamins, while 21 percent took vitamin C and 14 percent took vitamin E. Only 5 percent took daily doses of zinc, calcium or vitamin A.

Men who took daily doses of vitamins C or vitamin E had a 23 percent lower risk of prostate cancer, while those who took zinc daily decreased their risk by 45 percent.

While these results are suggestive, men should not change their use of dietary supplements based on this study alone, cautions Dr. Alan Kristal, who led the analysis of the supplement data and was the lead author on the vitamin-supplement paper.

"There are so few studies of dietary supplements and prostate-cancer risk that even with these results we still don't know enough about it to make any recommendations," says Kristal, a member of the Cancer Prevention Research Program in the Center's Public Health Sciences Division and a UW professor of epidemiology.

The results for the zinc supplements, which were unexpected, are consistent with earlier clinical studies that find much lower concentrations of zinc in cancerous prostate tissue compared to healthy prostate tissue. Zinc may play some role in regulating healthy prostate growth. The results for vitamin E on cancer risk were somewhat smaller than the protective effect found in one previous study.

Overall, Kristal says, the study adds weight to the idea that some supplemental vitamins and minerals may influence prostate-cancer risk.

The researchers suggest that further studies on dietary supplements and prostate-cancer risk should include a large population-based survey that incorporates blood-sample collection to better examine exposures to differing combinations and doses of supplements.