Dual role of selenium in health and disease

The relation between selenium and the etiology of human diseases is still partially unclear, appears to have a Janus-faced nature and is influenced by the chemical form of the element. While selenium was suggested to decrease cancer risk by observational studies and a randomized controlled trial (RCT), recent large RCTs showed no effect or even adverse effects of selenium on cancer risk, suggesting that the earliest studies were affected by exposure misclassification or unmeasured confounding. RCTs also showed no influence of selenium on cardiovascular risk and an adverse effect on diabetes risk. Conversely, RCTs indicated a beneficial effect of selenium on Keshan disease, a cardiomyopathy described in low-selenium areas in China, though the etiology of this disease is still not entirely elucidated. Selenium may also be involved in the etiology of neurological disease. This dual and intriguing activity of selenium on human health shown by epidemiologic studies is mirrored by laboratory studies. Thus, there is the need of a reassessment of what constitutes a safe intake of selenium in humans.