Selenium: How Much Is Too Much?

Selenium: How Much Is Too Much?
Selenium is an unusual and potentially dangerous substance. It is a required nutrient, but too much of it can be toxic. There are known cases of illness due to both too much and too little dietary selenium, and both conditions have been documented in China. The selenium status of groups of people is determined mainly by the amount of selenium in the soil where they live. Selenium gets into groundwater, and then into plants and animals, including humans. Selenium deficiency and toxicity are problems in livestock and have been studied for commercial purposes.

Selenium is available without a prescription in the United States in the form of dietary supplements, which are not regulated by the Food and Drug Administration anywhere near as strongly as drugs. Selenium in the diet is absorbed into the body very effectively, much more effectively than most other nutrients. Even when body stores of selenium are high, selenium absorption is very efficient, and toxic amounts can easily accumulate.

Toxicity associated with chronic selenium ingestion has been studied in laboratory animals, livestock, and humans. Most tests of chronic toxicity (of anything) focus on birth defects and cancer. The issue of birth defects has not been studied in humans, but in macaque monkeys, the experimental animal most closely related to humans of any animal studied, selenium does not cause birth defects. Selenium may actually prevent cancer, although the data are not conclusive.

Of course, one would like to know that selenium consumption is too high before adverse effects occur. Unfortunately, the amount of selenium which can be measured in body tissues is not related to selenium toxicity.

How then, is one to decide how much selenium is too much to consume? The best approach is probably to follow the advice of experts. The Food and Nutrition Board (FNB) of the United States has set the RDA (Recommended Daily Allowance) of selenium at 55 micro-grams for adult men and women, and the World Health Organization (WHO) has set the RDA for selenium at 70 to 350 micro-grams. The FNB has determined that the tolerable upper level (UL) of selenium intake per day is 400 micro-grams for adult men and women, while the WHO believes that toxic levels of selenium are approximately 40 times the RDA. Although these recommendations are based on admittedly insufficient data, they currently constitute the best scientific advice.

By Pauline Lerner

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