♦ Selenium has a vital role to play human health, especially with the immune system and even in combating cancers.
Selenium is an important nutritional trace element which has been causing considerable interest because when deficient, it results in Keshan and Keshan-Beck disease (Mo, 1987; Tan et al., 1987). It may also be a factor in preventing cancer (carcinogenesis). It also plays an important role in our immune system’s function, in thyroid hormone metabolism and in reproduction. It is also part of the body’s antioxidant defence system, preventing damage to cells and tissues.
A great variety of raw foods are sources of selenium – generally the intake is about 110 micrograms per adult (Bratakos et al., 1987). the trace element is usually found in proteins as a selenoamino acid but it can accumulate in the free form in plants. Surprisingly, some forms are volatile but most are water-soluble. Heat treatment can lead to losses of selenium compounds (Higgs et al., 1972) but such measures are not properly tabulated.
Protein rich foods are one of the best sources – meat, fish and eggs yielding up to 700 ng/g but not dairy products and vegetables (up to 80 ng/g) which are relatively low.
Selenium And The Immune System
Dietary selenium is essential for an optimum immune response. This mineral influences both the innate, “nonadaptive” and the acquired, “adaptive” immune systems. The innate immune system includes barriers to infection and non-specific effector cells such as macrophages. Both the T and B lymphocytes form the major effector cells of the acquired system that mature with exposure to immune challenges. Selenium-deficient lymphocytes are less able to proliferate in response to mitogens, and in macrophages, leukotriene B4 synthesis, which is essential for neutrophil chemotaxis, is impaired by its deficiency. These immune processes are improved by supplementation with this trace nutrient.
The humoral system is also affected by a deficiency in this trace nutrient; for example, IgG and IgM titres are decreased in humans. In endothelial cells from asthmatics, there is a marked selenium deficiency that results in an increase in expression of adhesion molecules, which causes greater adhesion of neutrophils.
Selenium In Proteins
This mineral is a key component of several functional selenoproteins required for normal health. It is found in enzymes such as glutathione peroxidases (GPx) and thioredoxin reductase, which protects vital components of the cell against oxidative damage. The glutathione peroxidases remove hydrogen peroxide and block the formation of harmful lipid hydroperoxides in the process. Selenium functions as a protective agent against toxic substances, and has a strong inter-relationship with Vitamin E.
This mineral can decrease the risk for some types of human cancers, especially those of the prostate, lung and colon. Selenium compounds possessing the Se-methyl selenocysteine moiety and gamma-glutamyl-Se-methyl selenocysteine have anti-cancer activities.
Bratakos, M.S., Zafiropoulos, T.F., Siskos, P.A. & Ioannou, P.V. (1987). Selenium in foods produced and consumed in Greece. J. Food Sci., 52, pp. 817-822.
Higgs, D.J., Morris, V.C. & Levander, O.A. (1972). Effect of cooking on selenium content of foods. J. Agric. Food Chem., 20, pp. 678-680
Mo, D.X. (1987). Pathology and selenium deficiency in Kaschin-Beck disease. In: Selenium in Biology and Medicine, part B (edited by G.F. Combs, Jr, J.E. Spallholz, O.A. Levander and J.E. Oldfield). New York: Van Nostrand Reinhold. Pp. 924-933.
Tan, J.A., Li, R.B., Zheng, D.X., Zhu, Z.Y., Hou, S.F., Wang, W.Y. & Zhy, W.Y. (1987). Selenium ecological chemicogeography and endemic Keshan disease and Kaschin-Beck disease in China. In: Selenium in Biology and Medicine, part B (edited by G.F. Combs Jr, J.E. Spallholz, O.A. Levander and J.E. Oldfield). Pp. 859-876. New York: Van Nostrand Reinhold