Selenium is a nutritionally essential trace element that naturally occurs in the soil. It enters our bodies through our food (both plant and animal products) and, to a lesser extent, water sources. Plants absorb selenium from the soil into their leaves, stems, seeds, and fruits. Animals eat plants containing the selenium and this is stored in their tissues. Some foods that are good sources of selenium are grains grown in the Midwestern U.S. and animal meats, particularly organ meats. Certain foods like brazil nuts are especially high in selenium.

The major functions of selenium in the body that have been discovered to date are as follows:

1. Selenium is found in the active site of many enzymes, such as thioredoxin reductase, which catalyze oxidation-reduction reactions. These reactions help to protect against cancer by encouraging cancer cells to undergo apoptosis. Apoptosis is a form of programmed cell death that occurs when genetic damage is detected in the cell, thus preventing the transfer of mutations to a future generation of cells.
2. An enzyme, Glutathione Peroxidase (or GSH-PX) requires selenium for its formation. This enzyme helps to prevent a process called oxidation, which may cause injury in cells.
3. Selenium appears to improve the functioning of the immune system and its response to infections.
4. Selenium appears to cause the formation of natural killer cells, which destroy foreign bacteria that enters the body.
5. P450 enzymes may be induced by selenium. These enzymes help detoxify some cancer-causing substances.
6. Selenium inhibits prostaglandins, which cause inflammatory reactions in the body.
7. Male fertility may be enhanced by selenium due to increased sperm motility.
8. At high doses, selenium may decrease the rates of tumor cell growth.

Selenium can be toxic at very high doses. This was first observed in animals that eat plants with toxic levels of selenium. Toxicity in humans has rarely been seen. However, toxicity has been reported in parts of the world where dietary selenium levels are extremely high. The reported symptoms of human toxicity from selenium have included increased nail and hair brittleness or crumbling, skin rashes, "pins and needles" sensations, irritability, nausea, vomiting, or garlic breath. These side effects were observed in approximately 10% of Chinese subjects consuming a diet with over 1,000 micrograms of selenium daily.

Content updated April 4th, 2003