Selenium is an essential trace mineral required for the structure of a number of metalloproteins in humans. In particular, selenium acts as a cofactor to the enzyme glutathione peroxidase, which catalyses the reaction between hydrogen peroxide and reduced glutathione to form water and oxidised glutathione. Poor selenium status is associated with an increased risk of cancer and selenium supplements decrease cancer risk significantly (here and here). The reason for this might relate to the free radical theory of disease as conceptualised by Denham Harman in the 1960’s. This theory suggests that a deficiency of endogenous or exogenous antioxidants leads to an internal milieu of increased oxidative stress and this causes tissue damage which in turn induces cellular changes that are the trigger for cancer (such as damage to DNA). Because poor selenium status is known to reduce tissue levels of the endogenous antioxidant enzyme glutathione peroxidase, low intakes of selenium may be a contributory factor in cancer development.
Certain regions of China are well known for their low selenium soils and because the populations in those regions eat local foods their diets are also low in selenium. The Henan Province and Molimo are areas with low selenium soils and research has reported high rates of cancer in the populations who live there. Because of this, a number of studies have investigated the effects of selenium supplements on the selenium status of populations who inhabit Molimo and the Henan Province. For example in one study1, researchers used supplements of selenite, an inorganic form of selenium, and selenomethionine, an organic form of selenium, to assess the effects of supplements on selenium status. The subjects were selected from the Molimo province of China because of their low selenium levels. The results showed that after eight weeks of supplementation with 150 μg per day selenium, plasma selenium increased compared to the placebo. Further, the selenomethionine was superior to selenite at raising plasma levels of selenium.
As has been shown previously, selenium supplementation was able to cause a rapid rise in plasma selenium, with increases in red blood cell selenium taking longer. This was mirrored by an initial increase in plasma glutathione levels within two weeks, followed by a rise in red blood cell glutathione within four weeks. However, over the course of the study, there was no difference between the ability of selenite or selenomethionine to cause increases in red blood cell or plasma glutathione levels, suggesting that both are equally effective at improving selenium status in in subjects with low selenium intakes. This explains the ability of selenium supplements to decrease cancer rates in areas of low selenium soil as has been reported in the nutritional literature (here). The fact that selenium supplements reduces the risk of cancer supports the contention that oxidative stress is the cause of, or a contributory factor in the aetiology of cancer, brought about by a reduction in the activity of glutathione in the tissues of the body.
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