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, and is sold in supplemental form 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). Supplementation was found to reduce viral load in individuals infected with human immunodeficiency virus (HIV) ⁽¹⁹⁾. It was also shown to reduce markers of inflammation and oxidative stress in patients with diabetic nephropathy ⁽⁴¹⁾, may improve glucose metabolism, but was not useful in preventing type 2 diabetes and in some cases may increase the risk ^{(20) (30) (42)}. In another study of asymptomatic older men, selenium alone or in combination with vitamin E was ineffective in preventing dementia ⁽⁴³⁾.

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) (36) and lung ⁽¹⁷⁾ cancers although findings from bladder cancer research are conflicting ^{(26) (38) (44)}. Selenium supplementation also resulted in reduction of side effects including hair loss, abdominal pain, and loss of appetite in ovarian cancer patients undergoing chemotherapy ⁽¹⁵⁾. It was found effective in reducing head and neck lymphedema ^(12) (14); to help reduce diarrhea associated with radiation therapy ⁽²⁴⁾ in gynecological cancer patients without influencing the effectiveness or long-term survival ⁽³⁹⁾; and may decrease chemotherapy-induced fatigue and nausea in adolescents ⁽⁴⁰⁾.

However, a recent review of randomized controlled trials did not find clear evidence of selenium’s cancer preventive potential ⁽²⁹⁾. 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, also failed to find evidence of protective effects of selenium. The trial was suspended in January 2009, when initial data analysis showed that selenium, by itself or when combined with vitamin E, did not prevent prostate cancer in healthy men ⁽²¹⁾. Further, a sub-group analysis failed to find any benefit for prevention of colorectal adenomas ⁽⁴⁵⁾. In other studies, selenium when used together with soy and Vitamin E, did not prevent prostate cancer progression ⁽²⁷⁾; and when used with lycopene, had no effect on modulating risk of prostate cancer ⁽⁴⁶⁾. 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⁾, and supplementation may increase prostate cancer mortality ⁽³⁷⁾.

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 increase the risk of certain types of skin cancer as well ⁽¹³⁾.

Additional studies indicate no association between dietary selenium intake and thyroid cancer incidence ⁽³⁵⁾, but that it may protect radiation-induced damage to salivary glands in patients with differentiated thyroid cancer ⁽⁴⁷⁾.