Moderate selenium levels tied to longer life

NEW YORK (Reuters Health) - When it comes to the effects of selenium on health and longevity, you may be able to get too much of a good thing. While moderate levels of the mineral are associated with longevity, once selenium levels climb past a certain mark, the odds of dying from any cause, or from cancer specifically, begin to tip upward, a new study suggests.

The study, of nearly 14,000 U.S. adults, found that higher blood levels of selenium were linked to a lower risk of death over 12 years, at which point the risk appears to increase.

The findings, published in the Archives of Internal Medicine, support earlier studies linking selenium to lower risks of prostate, lung and colon cancers. But the study also "raises the concern that high-normal levels of selenium may be associated with an increased risk of mortality," write the researchers, led by Dr. Joachim Bleys of Johns Hopkins University School of Public Health in Baltimore.

Selenium is a mineral that people require in small amounts; food sources include grains, certain nuts and some meats and seafood, such as beef and tuna.

The body incorporates selenium into proteins called selenoproteins, which act as antioxidant enzymes; antioxidants, in turn, help neutralize cell-damaging substances called free radicals. Some studies have linked higher selenium levels to lower risks of certain cancers and heart disease.

However, much of the research has been done in countries where people's selenium levels are often fairly low.

Most Americans, by contrast, get more than the recommended amount of selenium. While the recommended dietary allowance (RDA) is only 55 micrograms per day, the typical intake in the U.S. ranges from 60 to 220 micrograms daily.

For their study, Bleys and his colleagues used data for 13,887 U.S. adults who took part in a government health and nutrition study. They found that as the participants' blood levels of selenium increased, their odds of dying during the 12-year study period declined -- but only up to a point.

Once blood levels of selenium surpassed 130 ng/mL, the benefits stopped accruing; and once they passed 150 ng/mL, the odds of dying from any cause, or from cancer in particular, began to creep back up.

The findings caution against consuming too much selenium, according to the researchers.

"Most people in the U.S. obtain adequate selenium from their diet," Bleys told Reuters Health, noting that outright selenium deficiency is rare.

Given this, and the potential for adverse effects from high selenium levels, Bleys said there is "no rationale" for recommending selenium supplements to the general public.

It is not clear why high-normal selenium levels were tied to an increased death risk. But in theory, Bleys explained, excess selenium that is not incorporated into selenoproteins may actually generate free radicals rather than fight them.

SOURCE: Archives of Internal Medicine, February 25, 2008.