Selenium was discovered by the Swedish chemist Jo¨ns Jacob Berzelius in 1817 and has been recognized as an essential trace element for many life forms including man since 1957. As an essential trace element, the importance of selenium (Se) in humans is well established, and its deficiency has caused serious health effects in humans, such as Keshan disease. Foods are major natural source of Se, and its levels generally depend on soil Se levels. Since its discovery as an important component of antioxidant enzymes, such as glutathione peroxidase (GPx), thioredoxin reductase (TrxR) and iodothyronine deiodinases (IDD), there has been an increased interest in the study of other Se-containing proteins (selenoproteins) or enzymes (selenoenzymes)].Selenocysteine is recognised as the 21st amino acid, and it forms a predominant residue of selenoproteins and selenoenzymes in biological tissues. The molecular structure of selenocystiene is an analogue of cysteine where a sulphur atom is replaced by Se. Selenium can be measured in whole blood, blood fractions (plasma, serum, red blood cells), hair, nails, and urine. Plasma selenium levels below 0.6mM (40–50 ng/ml) are considered deficient, and risk of toxicity occurs at levels higher than 2mM (160 ng/ml), with reports of toxic effects at concentrations higher than 3mM (250 ng/ml)The increased production of ROS (reactive oxygen species) can exert oxidative stress in the physiological system, and if excess ROS are not properly regulated they can cause damage to cellular lipids, proteins and DNA. The damage caused by ROS has been linked to various human diseases, including heart diseases. The presence of ROS can also cause the oxidation of lowdensity lipoprotein (LDL),and it has been reported to be associated with initiation of atherogenesis in heart diseases. One hypothesis is that the presence of high Se as antioxidant selenoenzymes and selenoproteins may help to reduce the production of oxidised LDL and, therefore, would reduce the incidence of heart diseases.Thioredoxin reductase plays a significant role in preventing the development of atherosclerosis by reducing oxidative stress and increasing NO bioavailability