Selenium

Precautions

Selenium has a real risk signal. In a Danish trial, 300 µg per day for 5 years increased all-cause mortality ten years later; the authors recommend avoiding total intakes above 300 µg per day. At 200 µg per day over the long term, the risk of type 2 diabetes increased compared to placebo, especially in those whose baseline selenium levels were already highest. In people with a history of skin cancer, it increased the risk of squamous cell carcinoma. In a meta-analysis on thyroiditis, participants on selenium had significantly more adverse events. Finally, the limitations: it does not prevent cancer — the Cochrane review finds no benefit, either overall or for prostate, colon or lung cancer, and notes more high-grade prostate cancers in some studies; the large SELECT trial finds no protection against prostate cancer. The positive results of an older trial were limited to people with the lowest baseline selenium levels. It also makes no difference to cardiovascular diseases over 7.6 years of follow-up. On the thyroid, the drop in antibodies is real but its clinical significance is not demonstrated, and the Cochrane review judges the evidence to be of low to moderate quality.