Americans with diabetes have high levels of selenium in their bodies, prompting some health experts to suspect that it could contribute to development of the disease. In response to their new findings, a research team has recommended that U.S. residents stop taking supplements that contain selenium.
Most Americans ingest large amounts of the mineral—substantially more than people elsewhere—because soil in much of the country contains high levels that are absorbed by crops. Selenium occurs naturally in soil and leaches onto farm fields from irrigation and streams.

The research team, led by Johns Hopkins University epidemiologists, examined the diabetes rate and selenium levels of 917 people over the age of 40 who participated in a national health study conducted by the U.S. Centers for Disease Control and Prevention in 2003 and 2004. They found that most had a lot of selenium in their blood, but those with diabetes had substantially more.

The benefits and dangers of selenium have been debated in recent years because some studies show it might help protect people from cancer and heart disease. Selenium is an essential element and antioxidant, but medical experts say there is a fine line between the amount that the body needs and the amount that is harmful.

“Given the current diabetes epidemic, the high selenium intake from naturally occurring selenium in U.S. soil and the popularity of multivitamin/mineral supplements containing selenium in the U.S., these findings call for a thorough evaluation of the risk and benefits associated with high selenium status in the U.S.,” the researchers wrote in a study published online in Environmental Health Perspectives on May 15.

“Furthermore,” they wrote, “our findings suggest that selenium supplements should not be used in the U.S. until there is a better understanding of their potential risks and benefits.”

Supplements containing selenium have gained popularity in the United States because of anti-cancer claims, and selenium levels in people have been rising. Nearly one-quarter of Americans over the age of 40 take selenium supplements or multivitamin supplements that include selenium.

According to the U.S. Food and Drug Administration, selenium supplements are generally unnecessary because “normal consumption of food and water” provides adequate amounts. However, since 2003, the FDA has allowed manufacturers to state on labels that selenium “may reduce the risk of certain forms of cancer” based on “limited and not conclusive” evidence.

The new findings bolster the concerns of many health experts that extra selenium may be harmful.
"I would never, ever, ever take supplements with selenium in it," said Judith Stern, a professor of nutrition and internal medicine at University of California at Davis. “This study is not showing cause and effect. The association is provocative, not causal. But I still would never, ever use it. Selenium is toxic, and as Americans, we tend to really overdo it."
Maria Boosalis, director of clinical nutrition at the University of Kentucky's College of Health Sciences, recommended in a published report last year that "the indiscriminate use of selenium supplements should be approached with caution" until long-term health studies are conducted.
A link between diabetes and selenium initially was reported in 2007, based on results of people tested between 1988 and 1994 as part of the CDC’s National Health and Nutrition Examination Survey. Since then, average selenium levels in the country have increased 9 percent. Also in 2007, a large clinical trial in which people were given selenium tablets to see if it reduced their cancer risk was discontinued after they experienced a high diabetes rate.

The new study was directed by Dr. Eliseo Guallar, an associate professor of epidemiology at the Johns Hopkins University Bloomberg School of Public Health and a director at the National Center for Cardiovascular Research in Madrid, Spain.
Other recent research suggests that environmental exposures may play a role in diabetes. In particular, exposure to hormone-altering contaminants in the womb may lead to development of the disease later in life.

One new study, published online Monday in Environmental Health Perspectives , reported that mice exposed to dioxins in utero and then fed a high-fat diet developed higher blood glucose levels. Those fed a low-fat diet, as well as those that were overweight but not fed dioxins, did not have the same health effects. Dioxins are ubiquitous industrial pollutants.
The scientists in the dioxin study said their work with mice “demonstrates a clear interaction between diet and response to environmental chemicals.” One of the authors was Linda Birnbaum, an Environmental Protection Agency scientist who was recently named director of the National Institute of Environmental Health Sciences.

In the selenium study, diabetics (of all types*) had an average of nearly 144 parts per billion of selenium in their blood, compared with about 136 ppb for the non-diabetics. The highest risk of the disease was found for those with levels between 130 and 150 ppb.
People with the high selenium levels also had higher fasting plasma glucose and glycosylated hemoglobin levels, which indicate risk of diabetes. The researchers adjusted their findings to account for factors that might skew the link to diabetes, including age, obesity and smoking. Selenium levels actually decreased with higher body mass index.

Selenium helps the body produce antioxidants and also regulates thyroid hormones. But everyone in the study except for one person had selenium levels that exceeded 90 parts per billion, the maximum amount needed to maintain those healthy functions.

The researchers said the link between selenium and increased risk of diabetes may not apply to other populations with lower levels, such as most Europeans.
Stern said the only people who should take selenium are those who know they live in low-selenium areas, which are rare in the United States. A U.S. Geological Survey map shows high-selenium soil is scattered throughout the country. In China, however, selenium deficiencies are common so supplements are often necessary.

The recommended dietary allowance for selenium is 55 micrograms per day for adults, and most Americans reach that with diet alone. The FDA warns that the daily intake should not exceed 400 micrograms. The FDA recalled two popular dietary supplements last year that contained more than 40,000 micrograms, an amount considered toxic.

How the high selenium might cause diabetes is “largely unexplored,” Guallar and his co-authors wrote. It may increase insulin resistance.

“Given the high selenium exposures in the U.S., the mechanisms underlying these associations need to be investigated,” they wrote.

This article originally ran at Environmental Health News , a news source published by Environmental Health Sciences, a nonprofit media company.

* Editor's Note: This sentence was changed to clarify what type of diabetes the study examined in response to questions from our commenters.