Selenium is an integral part of thioredoxin reductase and the glutathione peroxidases and therefore is intimately involved in the body’s antioxidant systems. These enzymes are involved in controlling tissue levels of free radical molecules and maintain cell-mediated immunity.
Chemoprotective effects of selenium have been indicated by an epidemiological relationship, RCTs and by experimental studies of selenium and known carcinogens in the development of specific cell lines. Overall, it appears that selenium works by inhibiting important early steps in carcinogenesis.
Several mechanisms have been postulated to explain the chemopreventative effect of selenium, including protection against oxidative damage, alterations to immune and metabolic systems, alterations to carcinogen metabolism, production of cytotoxic selenium metabolites, inhibition of protein synthesis, stabilisation of genetic material and stimulation of apoptosis. One study demonstrated that combining vitamin E succinate and methylselenic acid produces a synergistic effect on cell growth suppression, primarily mediated by augmenting apoptosis.
In humans, the chemopreventative effect is strongest for individuals with the lowest selenium status; however, it is still unclear whether low selenium status is implicated in the aetiology of cancer or whether it produces a state of increased susceptibility to the effects of carcinogens.
Confirmed in both animal studies and human trials, immunomodulation is in part due to improved activation and proliferation of B-lymphocytes and enhanced T-cell function. Interestingly, selenium concentrations significantly decrease during stages of acute infection, suggesting increased use and/or excretion or decreased absorption during this period.
THYROID HORMONE MODULATION
Selenium is required for normal thyroid hormone synthesis, activation and metabolism. Three different selenium-dependent iodothyronine deiodinases (types I, II, and III) can both activate and inactivate thyroid hormone, making selenium an essential element for normal development, growth and metabolism through the regulation of thyroid hormones.
Selenium: Other Actions
Selenium is required for testosterone synthesis, normal sperm maturation and sperm motility. Two clinical studies have confirmed this association and identified selenium supplements as able to increase sperm motility.
Selenium deficiency produces a significantly increased COX-2 protein expression, as well as higher PGE2 levels, according to one in vitro study. It has also been theorised that selenium may decrease leukotriene production. In vivo tests have identified anti-inflammatory activity in the lung with selenium, which is thought to relate to an increase in glutathione levels and immune parameters.
REDUCES HEAVY METAL TOXICITY
Selenium protects against toxicity of some heavy metals such as cadmium, arsenic, lead, silver and mercury. A physiological role for selenium in counteracting heavy metal poisoning has been proposed. It appears that the form of selenium is important, as inorganic selenium has been shown to enhance the toxic effects of inorganic arsenic by increasing its retention in tissues and suppressing its metabolism in vitro.
Selenium supplementation reduces high-fat diet induced atherosclerosis, according to an in vivo study.