Tissue levels of selenium are influenced by dietary intake and the geochemical environment. Selenium, as selenomethionine or selenocysteine, is widely distributed in the body. The antioxidative effects of selenium and vitamin E may reinforce each other by the overlap of their protective actions against oxidative damage.
Selenium deficiency takes years to develop when food intake is inadequate. Cases of severe deficiency have only been reported for regions in China, where the soil has an exceptionally low content of the mineral. Likewise, Keshan-Beck disease has been found in Mongolia in preadolescent and adolescent children also due to low amount of selenium in the soil. A relatively low intake of selenium may increase the risk of cancer . Evidence of selenium toxicity has been reported in China and Australia.
Chromium was deemed essential when patients receiving TPN exhibited abnormalities of glucose metabolism that were reversed with chromium supplementation. Chromium potentiates insulin action and, as a result, influences carbohydrate, lipid, and protein metabolism. The proposed role of chromium with the so-called glucose tolerance factor (GTF) is controversial. Chromium, similar to zinc, may regulate gene expression. Chromium deficiency can result in insulin resistance and some lipid abnormalities. Cases of toxicity have been reported in athletes and power lifters taking chromium picolinate supplements in high doses.
Other Trace Elements
There is evidence that manganese, molybdenum, vanadium, boron, arsenic, nickel, and silicon may be essential in humans. These elements may be necessary in extremely small amounts but can be toxic in