A new study suggests that the mineral selenium may be important in reducing prostate cancer risk as men age, according to a report in the Journal of Urology (Vol. 166, No. 6: 2034-2038).

The research suggests that the older men get, the less selenium they are likely to have in their blood. This may explain why a man's chance of getting prostate cancer goes up as he ages, since there may be a relationship between very low selenium levels and increased risk of prostate cancer.

“Our results suggest the possibility that selenium supplements may be especially beneficial for older men,” said lead author, James D. Brooks, MD, assistant professor of urology at Stanford University Medical Center in Palo Alto, Calif.

Brooks cautioned that too much selenium can be toxic. The Institute of Medicine, a government organization that determines the recommended dietary allowances (RDA) of vitamins and minerals, suggests a daily selenium intake of 55 micrograms (mcg) daily for men over the age of 14. Selenium is found in organ meats (liver), seafood and vegetables (which depend on the selenium content of the soil). The maximum intake daily of selenium should not exceed 400 mcg daily from all sources.

Most Americans get enough selenium, Brooks noted, but in some areas of the US there is not much selenium in the soil, so there may not be enough in drinking water or in food.

To study the relationship between selenium and prostate cancer, Brooks and colleagues compared the levels of selenium in the blood from 52 men taken before they developed prostate cancer, to the selenium levels in blood given by 96 men who did not later develop prostate cancer.

These men were all part of an ongoing study called the Baltimore Longitudinal Study of Aging. They were examined about every two years over a long period of time.

Men with the least amount of blood selenium were four to five times more likely to develop prostate cancer in the next few years than those with more, the researchers found.

“There may be a threshold level — a certain amount needed to lower risk — but beyond that, adding more selenium may not offer any more protection,” said Brooks.

An American Cancer Society (ACS) expert on cancer and nutrition said the Stanford study confirms earlier studies that show selenium may reduce risk of prostate cancer by as much as 60%, and it adds new information as well.

“This is the first study to show that selenium levels may drop as age increases, which could help explain why men’s chances of developing prostate cancer go up as they get older,” said Carmen Rodriguez, MD, a senior epidemiologist in the ACS department of Epidemiology and Surveillance Research.

But Rodriguez cautioned that the study had so few men in it that it’s difficult to be sure that conclusions from it are meaningful.

And she expressed concern that blood samples weren’t necessarily collected at the same ages from men who developed prostate cancer later and from those who didn’t, making the comparison not as equal as a better matched one.

Rodriguez said men interested in those questions and in learning more about reducing their risk of prostate cancer with selenium and/or vitamin E may want to participate in the SELECT Trial trial, or wait for its outcome, she noted.

“In the meantime, all men can use the knowledge we already have to keep their prostate cancer risk as low as possible by learning the risk factors for prostate cancer, and how to actively reduce their chances of developing the disease,” noted Rodriguez.