Men with higher selenium levels may be at lower risk of aggressive prostate cancer (PCa), according to a new study.

Each 80th percentile increase in blood and nail selenium concentrations was associated with a significant 57% and 82% decreased odds of aggressive PCa, respectively, after adjusting for body mass index, age, height, smoking status, education, and marital status, Naomi E. Allen, MSc, DPhil, of the University of Oxford in Oxford, UK, and colleagues reported in the Journal of the National Cancer Institute (2016;108:djw153). The researchers defined aggressive PCa as advanced stage disease and/or PCa-related death.

The concentration of selenium in nail, but not blood, was inversely associated with the risk of total PCa. Each 80th percentile increase in nail selenium associated with a significant 71% decreased odds of total PCa and 67% decreased odds of non-aggressive PCa.

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“Because a substantial proportion of PSA-detected cancers remain biologically indolent for many years, the identification of factors that are associated with the development of clinically aggressive cancers is important,” Dr Allen’s group wrote. “Hence, our finding that both blood and nail concentrations were associated with a lower risk of aggressive prostate cancer is of potential etiological relevance.”

The results are based on an analysis of individual participant data from 15 prospective studies in which data were available for 4,527 case patients and 6021 control patients for blood selenium and 1970 cases and 2086 controls for nail selenium.

The researchers acknowledged that it is possible that the inverse association of blood and nail selenium level and risk of aggressive PCa might be due to confounders, but noted that they adjusted their analyses for a range of lifestyle factors as well as the matching factors used in the individual studies, “none of which materially influenced the risk estimates.”

Dr Allen and her collaborators pointed out that nails are a more reliable marker of long-term selenium exposure, and this might explain why nail, and not blood, selenium levels are inversely associated with the risk of total PCa. Nail clippings provide a measure of exposure over several weeks and up to 12 months prior to sample collection, they noted. Blood levels reflect shorter-term (1–2 weeks) selenium exposure.

According to the researchers, the absence of an association between blood selenium levels and total PCa risk is consistent with the findings of the Selenium and Vitamin E Cancer Prevention Trial (SELECT). In this phase 3 trial, investigators randomly assigned 35,533 men to 1 of 4 groups: placebo plus placebo; vitamin E plus placebo; selenium plus placebo; and selenium plus vitamin E. The median follow-up was 5.5 years. Selenium was dosed at 200 µg/day and vitamin E was dosed at 400 IU/day. Selenium or vitamin E, alone or in combination, did not significantly affect PCa risk.

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