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
