Selenium is most concentrated in the thyroid and helps regulate its function. Severe selenium deficiency may impair its function and is linked to different thyroid disorders.
The benefits of fish consumption are often attributed to polyunsaturated fatty acids, omega-3s, but high amounts of selenium found in fish may be equally important. Selenium works in synergy with the fatty acids, preventing their breakdown and enhancing their beneficial effects on cognition.
The role of selenium in cancer prevention is a hot topic of debate and the findings are conflicting. Selenium supplements aren’t approved for cancer prevention or treatment. Ever since the 70s, studies have been reporting that increased selenium intake reduces cancer deaths and vice versa. In one study, the dietary intake of selenium in 27 countries was linked to fewer deaths from cancer. In the US, deaths from cancer were much higher in low selenium counties, in which people are more likely to be deficient.
Selenium deficiency impairs immune function. Accordingly, selenium supplementation stimulates the immune system, even in people who are not deficient. Lymphocytes of people who supplemented with selenium (200 µg/day) were more active and efficient at destroying foreign invaders and tumor cells.
Balanced selenium levels are important for reproductive health in both men and women. In men, selenium is important for producing sperm and testosterone.
Selenium may protect against heart disease, probably by boosting the antioxidant glutathione, reducing oxidative stress and preventing platelet clumping.
Low selenium levels have been linked to depression, anxiety, tiredness, and mental confusion. In one study of 50 people, selenium supplementation (100 mcg/day) over 5 weeks improved mood, energy levels, and lessened anxiety.