Selenium is an essential trace element required as a cofactor for a number of selenoproteins. The most well known and most extensively researched selenoprotein is glutathione peroxidase, a family of antioxidant enzymes that catalyses the removal of lipid peroxides to less damaging alcohol derivatives. In this role glutathione peroxidase is the primary water soluble antioxidant in the cells, where it interacts with other antioxidants (here) to maintain favourable oxidation levels. Interest in the nutritional role of selenium has increase since the seminal paper by Clark et al (here) showed a 50 % reduction in the mortality rate from cancer with a 200 μg selenium tablet. Selenium is hypothesised to prevent cancer because of its role as an important antioxidant in cells, something which may protect the cells from incurring DNA damage and ultimately turning cancerous. Low selenium intakes are increasingly being linked to the development of cancer, and regions of China such as the Henan province have been studied because they have a high incidence of cancer and also are characterised by low selenium soils.
Clark used a high selenium yeast in his study, an organically bound form of selenium synthesised by growing yeast cells. Plants also create organically bound selenium during growth. Organically bound selenium refers to the binding of the selenium ion to an organic molecule. In many cases this organic molecule is an amino acid, most often cysteine or methionine. Evidence suggests that such organically bound selenium is more bioavailable in humans compared to consumption of the inorganic metal ions such as selenite or selenate. These latter two forms are often found in supplements, particularly multivitamin tablets as they are cheaper to produce and easier to manufacture into a tablet. Studies have investigated the bioavailability of selenium in humans by comparing organically bound selenium in foods and organically bound selenium from yeast in supplements with the inorganic sodium selenite form of the vitamin1. Subjects were all healthy Finnish males who took selenium at a dose of 200 μg per day for a period of eleven weeks.
The results of the study showed that those men ingesting their selenium in the form of selenium rich wheat or selenium rich yeast had gradual increases in plasma levels of selenium for the entire eleven weeks of the study (levels of plasma selenium reached 175 ng/mL at the studies end). However, those taking the sodium selenate had a plateau in their plasma levels of selenium that remained at 110 ng/mL following four weeks of supplementation. Prestudy levels of plasma selenium were 70 ng/mL, so all subject experienced significant increases in plasma levels from their original poor selenium status, but the organically bound selenium was superior. Wheat and yeast forms of selenium caused rapid increases in platelet glutathione peroxidase activity over four weeks before plateauing. However, levels rose more slowly in the sodium selenite group. Ten weeks after supplementation ceased, the wheat and yeast groups still had higher plasma selenium concentrations compared to baseline, but the sodium selenite group did not (figure 1).
Figure 1. The changes in the plasma levels of selenium after supplementation with high selenium wheat, high selenium yeast or sodium selenite.
Dr Robert Barrington’s Nutritional Recommendation: Selenium is increasingly being seen as deficient in the diet of most individuals in Western nations. This may explain the high rates of cancer in these countries. It is always better to try to obtain essential nutrients in the diet, as this is the form that is most recognised by our own biochemistry. However, it is often a lottery as to whether foods contain adequate selenium without expensive laboratory tests. Thus taking a 200 μg selenium tablet every day makes sense, as this was the dose shown by Clark et al to reduce the risk of cancer mortality by 50 %. Selenium bound to organic molecules as found in selenium yeast is the prefered choice, but even selenite and selenate show beneficial effects. Because selenite and selenate are less bioavailable higher intake may be required to maintain optimal glutathione peroxidase concentrations.
RdB
1Lavender, O. A., Alfthan, G., Arvilommi, H., Huttunen, J. K., Kataja, M., Koivistoinen, P. and Pikkarainen, J. 1983. Bioavailability of selenium to Finnish men as assessed by platelet glutathione peroxidase activity and other blood parameters. American Journal of Clinical Nutrition. 37: 887-897
