Selenium, an essential dietary trace mineral, is an important component of cellular antioxidant defense systems that include enzymes such as glutathione peroxidase ⁽¹⁾. 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 sold in supplemental form to boost immune function and for disease prevention.

Studies on the role of selenium in reducing cardiovascular disease risk are inconclusive ^(23) (28), although a meta-analysis suggests benefit with antioxidant mixtures when they contain selenium ⁽⁴⁸⁾. Data are also mixed on whether supplementation can reduce viral load in patients with HIV ^(19) (49). Although it may improve glucose metabolism and reduce markers of inflammation and oxidative stress ⁽⁴¹⁾, supplementation is not useful in preventing diabetes and in some cases may increase the risk ^{(20) (30) (42) (50)}. Selenium alone or in combination with vitamin E also did not prevent dementia ⁽⁴³⁾.

In studies on the relationship between selenium and cancer prevention, data suggest it may help prevent gastrointestinal ^(16) (36) and lung ⁽¹⁷⁾ cancers, although findings from bladder cancer research are conflicting ^{(26) (38) (44)}. Other studies on whether selenium status impacts cancer risk are also mixed ^{(51) (52) (53)}, and a review of RCTs did not find clear evidence of cancer preventive potential ⁽²⁹⁾.

In addition, the Selenium and Vitamin E Cancer Prevention Trial (SELECT) failed to find protective effects against prostate cancer ^(8) (21) or colorectal adenomas ⁽⁴⁵⁾. In other studies, selenium along with soy and vitamin E did not prevent prostate cancer progression ⁽²⁷⁾, and selenium taken with lycopene did not modulate prostate cancer risk ⁽⁴⁶⁾. Moreover, 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 non-small cell lung cancer did not show benefit, but did suggest a possible nonsignificant increase in secondary lung cancers ⁽²⁵⁾. Long-term supplementation may increase the risk for certain types of skin cancer as well ⁽¹³⁾.

With respect to dietary intake, no association between dietary selenium and thyroid cancer incidence was found ⁽³⁵⁾.

A few studies suggest selenium supplementation may reduce some cancer treatment side effects including hair loss, abdominal pain, loss of appetite ⁽¹⁵⁾, diarrhea ^(24) (39), fatigue and nausea ⁽⁴⁰⁾, head and neck lymphedema ^(12) (14), and salivary gland damage ⁽⁴⁷⁾. However, more studies are needed and supplementation in patients with low levels or during active treatment should be under the guidance of healthcare practitioners in cancer populations.