2.10 Selenium (Se)
Selenium is found in the body in an important group of enzymes (glutathione peroxidases) which have important antioxidant properties and work in conjunction with vitamins C and E to destroy free radicals in cells. Some studies have shown that a higher selenium level is linked to a lower risk of breast, prostate and colon cancer, which may in part be due to selenium's antioxidant function. Other selenium-containing proteins help to regulate thyroid function and play a role in the immune system.
The selenium content of foods depends partly on the protein level, since selenium is found attached to the amino acid cysteine in animal proteins and to methionine in plant proteins. However, the level in food also depends on the selenium content of the soil where the plants are grown or the animals are raised. Levels are low where soils are acid and there is heavy rainfall. Soils in some parts of China and Russia have very low amounts of selenium and selenium deficiency is often reported in those regions because most food in those areas is grown and eaten locally. Selenium is found in fish, meat and eggs and in bread, though the level depends on the source of the wheat. Much of the wheat from America and Canada contains sufficient selenium, but when bread-makers in the UK switched from Canadian to European wheat, the selenium levels in the wheat were found to be 10 to 50 times lower, resulting in a significant fall in the daily intake of selenium in the UK. At least one bread manufacturer subsequently added selenium to their products. An alternative solution is to encourage farmers to use fertilizer containing added selenium, on their land. In the UK, Brazil nuts are the richest dietary source of selenium and if eaten in large quantities, could result in excess intake.
Selenium deficiency may contribute to the development of arthritis, coronary heart disease, thyroid malfunction, and to a weakened immune system. Evidence suggests that selenium deficiency does not usually cause illness by itself, but makes the body more susceptible to illnesses caused by other nutritional, biochemical or infectious stresses. Keshan Disease, named after the area of China where it was originally found, is a specific disease associated with selenium deficiency, resulting in an enlarged heart and poor heart function in children. It was first described in the early 1930s, though now has largely been eliminated due to a more varied intake of food.