OREGON STATE UNIVERSITY

Scientists downplay selenium toxicity, explore deficiencies

08/12/1997

CORVALLIS - Oregon State University scientists who just developed a better method for assessing selenium metabolism say concerns about the trace mineral's toxicity are usually unwarranted.

"You'd have to consume more than 60 of the usual-strength (50 micrograms each) pharmaceutical selenium tablets a day for years before you'd even get sick," said Philip Whanger, an OSU professor of agricultural chemistry.

Still, the improved toxicity test Whanger and colleagues developed is important because there are incidences where humans and animals get overdosed. In the 1980s, for example, a now-defunct pharmaceutical company produced tablets that contained 1,000 times more selenium than advertised. There have also been cases where injections into farm animals exceeded recommended doses tenfold.

Research is also being done on the reverse side of the problem - selenium deficiency.

Whanger's selenium toxicity assay method uses albumin, a blood protein, as a marker. A. ratio of selenium to albumin exceeding nine nanograms per milligram in the blood plasma is toxic.

The assay's accuracy and precision makes finding a needle in a haystack seem like child's play. There are a 28 billion nanograms, or 28 thousand milligrams, in an ounce. In humans, there are less than three drops of selenium in an individual's 10 liters of blood. "It's like measuring a shot-glass of vermouth in a railroad tank car of gin," Whanger joked.

While there are a few places in China in which high-selenium soil produces crops toxic to the humans and animals that eat them, most of the world has normal selenium levels.

A more significant problem, the researchers say, is selenium deficiency.

Soil in parts of the Pacific Northwest, the upper Midwest and the northeastern United States, New Zealand and China are known to be deficient in selenium. Prior to the 1970s, selenium deficiencies caused thousands of deaths in China, particularly to children and young adults. Selenium-related deaths seldom occurred after scientists there identified the problem and started putting selenium in table salt.

Whanger said some soils in the Pacific Northwest are among the lowest in selenium of any areas in the United States. In some years early in the century, a disorder called"white muscle disease," which is caused by selenium deficiency, killed as many as half of the lambs and calves born to ewes and cows raised on crops grown in Oregon.

OSU researchers started working with farmers and ranchers on the cause and treatment of white muscle disease in the 1920s. By 1960, white muscle disease was eliminated and researchers estimated that the Oregon livestock industry saved $10 million a year by giving selenium supplements to their animals.

There is strong evidence that selenium supplements are good for humans as well, according to Whanger. A recent study by Cornell University scientists showed several selenium compounds reduce the incidence of prostrate, colorectal and lung cancer. Whanger said he takes a 100-microgram selenium tablet each day, and his colleague, Judy Butler, senior research chemist on the OSU selenium project, took selenium tablets when she was pregnant.

"Selenium is only needed in very small amounts, but it is essential for animal life," she said.

Butler's studies of pregnant women in Oregon and New Zealand showed selenium levels decline during pregnancy until birth, when they rise again to normal levels. Selenium supplements maintained normal levels throughout pregnancy until just prior to delivery.

She and Whanger said similar patterns were found in Chinese women suffering from selenium deficiency. Their children suffer from Keshan disease, a cardiomyopathy heart ailment.

"The problem is greatest after the children have been weaned," Butler said. "Until then the mother's milk, which contains a beneficial enzyme, offers some protection to the child. But the mother's selenium level goes dangerously low. In effect, the mother sacrifices her own body reserves to put the enzyme in the milk for her child."

The enzyme is glutathione peroxidase (GSH-Px). It converts peroxides to other compounds that are not harmful. Peroxides are breakdown products of fatty acids that damage cell membranes, causing rapid aging and damage to the skin. Selenium supplements reduce these problems, Whanger said.