17.4: Selenium

It was selenium’s toxic effects that first attracted nutritionists’ attention to this trace mineral. When animals grazed on plants high in selenium, they came down with a disease called blind staggers; they became stiff and lame, and then became blind and paralyzed before they died.

So it came as a surprise when this “potent poison” was later found to be an essential nutrient. Selenium is part of an enzyme that acts as an antioxidant. (Vitamins C and E also act as antioxidants, as discussed in earlier chapters.) Selenium appears to have other less-defined functions as well.

Selenium is another of the elements in which soil content influences food content. Early reports of both selenium toxicity and deficiency due to soil levels came from rural China. (Rural China has been a gold mine for studies of diet and health, because the people there tend to live their entire lives in one area, and eat locally grown foods that can vary substantially from one area to the next.)

In the 1960s there was a major outbreak of selenium toxicity in several rural villages in the Hubei Province of China.6 It started with a drought that led to a failure of the rice crops. More vegetables and corn were eaten instead. Although the rice, vegetables, and corn were grown in local soil heavily contaminated with selenium-rich coal, the rice had a lower selenium content. So the switch from less rice to more vegetables and corn meant more selenium in the diet.

The most common symptom was a loss of hair and nails. Other symptoms such as skin lesions, more tooth decay, and abnormalities of the nervous system were also thought to be the direct result of selenium toxicity. (It’s hard to pinpoint the symptoms to the selenium toxicity with certainty because the drought brought about other dietary and environmental changes as well.)

In 1979, there was a report of an epidemic in the Keshan province of China of what became known as Keshan’s disease, a disease of the heart muscle, characterized by an enlarged heart, abnormal heart rhythms, and, sometimes, heart failure and death. The soil there was very low in selenium, resulting in low selenium in the crops. Selenium supplements cleared most of the symptoms. (Deficiencies of other trace minerals and additional stresses may also have contributed to the disease.)

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Selenium supplements taken in large doses can cause “garlic breath”—perhaps a safe-guard against a temptation to take too much.

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Some studies suggest that selenium deficiency increases the risk of cancer, and some people have responded by self-prescribing selenium supplements. Large doses of selenium supplements can be extremely toxic.

Worldwide, the selenium content of plant and animal food largely depends on where the plants are grown and where the animals are raised. Meat has some selenium because it’s a required nutrient for animals. Plants, however, don’t need selenium, so content reflects the selenium in the soil.

People who don’t eat meat because of choice or poverty, and only eat local plant foods grown in low-selenium soil are most at risk for deficiency. In the U.S., animals raised for food have controlled, selenium-containing diets, and we get plant foods from such a variety of locations/soils that selenium deficiency or toxicity is unlikely.