Selenium, an essential dietary trace mineral, is an important component of antioxidant systems such as glutathione peroxidase (1) that neutralize and protect against damage caused by free radicals and reactive oxygen species. It also plays an important role in thyroid function. Selenium can be obtained from the diet by consuming whole grains, meats, seafood, poultry and nuts. It is also marketed as a supplement to boost immune function and for prevention of cardiovascular, rheumatic diseases, and cancer.
Several studies have been conducted to determine the role of selenium in reducing the risk of cardiovascular disease but data are inconclusive (23) (28).
Selenium supplementation was found to reduce the viral load in individuals infected with human immunodeficiency virus (HIV) (19). Studies also indicate that selenium may improve glucose metabolism but it was not useful in preventing type 2 diabetes and in some cases may increase the risk (20) (21) (30).
The importance of selenium in cancer prevention has been documented in epidemiological studies and clinical intervention trials. Data suggest its benefits in preventing gastrointestinal (16), lung (17) and bladder cancers (26). Selenium supplementation resulted in significant reduction of side effects including hair loss, abdominal pain, and loss of appetite in ovarian cancer patients undergoing chemotherapy (15). It was also effective in reducing head and neck lymphedema (12) (14) and diarrhea associated with radiation therapy (24).
However, the large prevention study SELECT (Selenium and Vitamin E Cancer Prevention Trial) (8), based on previous data indicating that selenium and vitamin E reduced the incidence of prostate cancer, did not find evidence of protective effects of selenium. The trial was suspended in January 2009, when initial data analysis showed no reduction in the risk of prostate cancer with selenium alone or with vitamin E (21). Another study showed selenium, when used together with soy and Vitamin E, did not prevent prostate cancer progression (27). Moreover, results from a cross-sectional analysis of men with prostate cancer indicate that selenium levels may influence the risk of aggressive prostate cancer (22). Further, a review of randomized controlled trials did not find any evidence of selenium's cancer preventive potential (29).
A chemoprevention trial of selenium in patients with NSCLC (non small-cell lung cancer) was also halted after interim analysis as data did not show any benefit of selenium supplementation; there was an increase in secondary lung cancers in selenium users, although it was statistically insignificant (25).
Long-term use of selenium may also increase the risk of certain types of skin cancer (13).