Too much or too little selenium associated with higher mortality

Selenium supplements are beneficial to people with low levels of the mineral, but those with levels over 122 µg/L do not obtain improvement in mortality or other outcomes from supplementation, according to a new review of available studies on selenium.

A large majority of the U.S. population already meets the 122 µg/L cutoff, according to the review. Conclusions appeared online at The Lancet on Feb. 29.

Prospective studies included in the review found a relationship between low overall mortality and high selenium concentrations, while low selenium was an independent predictor of all-cause mortality in other studies. However, the author noted, “Such studies are prone to confounding since plasma selenium concentrations are higher in fit and well-nourished elderly people than in those who are frail, poorly nourished and unwell.”

Selenium has immunostimulant effects, including enhancing proliferation of activated T cells and other effects, the review said. But evidence outside of in vitro and animal studies is scarce, the reviewer noted. Only one study looked at a functional outcome, in which patients taking supplements cleared an oral, live, attenuated poliovirus more rapidly than those given placebos.

While selenium has cardiometabolic effects, randomized trials have not shown a cardioprotective effect, the reviewer noted. However, a meta-analysis of 25 observational studies showed a significant inverse association between selenium concentrations and risk of coronary heart disease, particularly in low-selenium populations.

Prospective studies have shown some benefit on the risk of bladder, colorectal, liver, esophageal, gastric, thyroid and prostate cancers. In particular, prostate cancer studies showed “more significant protective associations are consistently detected between selenium and risk of advanced, rather than localised or low-grade, prostate cancer,” especially among smokers, the author wrote.

Evidence is conflicting between studies looking at selenium concentrations and glucose metabolism, the review concluded. Lower selenium concentrations seen in the studies might be an effect of diabetes, or high selenium levels might have an effect on insulin signaling processes.

The reviewer wrote, “The crucial factor that needs to be emphasized is the inextricable U-shaped link with selenium status: additional selenium intake (e.g. from food fortification or supplements) may well benefit people with low status. However, people of adequate or high status could be affected adversely and should not take selenium supplements.”