Selenium and immune function: a systematic review and meta-analysis of experimental human studies

Nine trials of selenium supplementation in healthy people (5 in North America, 4 in Europe), lasting 8 to 48 weeks and using both inorganic and organic selenium, were pooled. Selenium did not meaningfully change immunoglobulin or white blood cell concentrations, and dose-response analysis showed that raising plasma selenium above 100 micrograms/L produced no further increase in IgA or T cells. Natural killer cell counts followed an inverted U-shape, being lower both below and above 120 micrograms/L, suggesting possible harm at higher intakes. Cytokine levels were essentially unaffected; the only benefit found was increased natural killer cell lysis activity. The authors conclude the evidence does not support supplementing selenium above recommended dietary intake for immune function.