Q. I have read that selenium might be helpful for preventing certain cancers, but that it can also be highly toxic. How much is enough and how much is too much?
A. An excess of any nutrient can be toxic, but minerals especially tend to have narrow limits between too much and not enough. This “risk/benefit” ratio can be very high in vitamins, 200 or even higher, but as low as eight for minerals such as selenium. In fact, until the late 1950s selenium was believed to be a toxic element with no nutritional value. The status of selenium as a nutrient was established in 1973, with the discovery of an enzyme (glutathione peroxidase) that is necessary for life, which contains selenium.
When it was agreed that selenium was an essential nutrient, the discussion began over the amount needed for optimum health. Because of its small safety factor there is very little room for error in the calculations for selenium requirements compared to most other nutrients. For Americans, the adult Recommended Dietary Allowance, set by the Food and Nutrition Board, is 55 micrograms per day, and the Tolerable Upper Intake Level, or maximum intake, is 400 micrograms/day. This RDA may seem low compared to the 100-200 micrograms/day found in many supplement formulas, but the uneven distribution of selenium in the soil in various parts of the country makes this necessary.
In general, soils in the Pacific Northwest, upper Midwest, the New England states, the Atlantic coast, and the Florida peninsula have low levels of selenium while soils in North Dakota, South Dakota, Colorado, and Western Nebraska tend to have an excess. Most of mid-America benefits from appropriate soil levels of selenium, and areas where selenium is deficient far exceed those areas where selenium toxicity is a problem.
Plants grown where the soil is rich in selenium contain much higher than average amounts of the mineral. Combining selenium from food with a large selenium supplement might lead to selenium excess, which can cause hair loss and fingernail/toenail damage. On the other hand, eating plants grown in low-selenium soil may lead to selenium deficiency.
There have been a number of studies on the relationship between selenium deficiency and cancer of various sorts, most of them verifying the protective effect of selenium against cancer. New research suggests that selenium may be even more beneficial to people with a certain gene. Comparing this gene in healthy, cancer-free people with the gene from breast cancer tissue samples, researchers found that one version of the gene was associated with a higher breast cancer risk. This same gene was also less responsive to selenium stimulation, meaning people with this gene require a higher dose of selenium to achieve a protective effect against cancer. Thus, genetic testing may someday allow use of higher selenium supplements for those who would benefit most.
Unless you live in an area of very high soil selenium, a supplement of 70-100 micrograms/day (or even 200 micrograms/day if you live in a low soil selenium area) seems entirely prudent. If a pharmaceutical company came out with a drug that could reduce the risk of cancer as much as selenium supplementation appears to, you would soon find full-page ads in newspapers and magazines telling you to “ask your doctor for this new drug.”
This article appears in August 20 • 2004.