Review Finds Evidence That Selenium Supplements May Increase the Risk of Type 2 Diabetes
By Brenda Goodman, MA
WebMD Health News
Latest Diabetes News
Reviewed by Louise Chang, MD
Feb. 28, 2012 -- Taking selenium? You may not need to. There's new evidence to suggest that selenium supplements aren't necessary for most Americans. They may even cause harm.
And if you pop a daily multivitamin, as more than one-third of Americans do, check the label. Many multivitamin and mineral formulas contain selenium.
"It isn't always that more is better. More often, 'more' isn't better. Really, in terms of selenium, that was one of the points I wanted to bring out," says researcher Margaret P. Rayman, DPhil, a biochemist at the University of Surrey in the U.K.
In a research review published in The Lancet, Rayman concludes that most Americans get enough selenium in their diets.
And a few studies included in the review suggest that taking more selenium in supplements may increase the risk for type 2 diabetes, though evidence is conflicting on that point.
Experts who were not involved in the study agree that most Americans shouldn't be taking extra selenium.
"There is no evidence that selenium supplementation of the U.S. population would be helpful," says Raymond F. Burk, MD, a professor of medicine at Vanderbilt University in Nashville, Tenn.
"In fact, there have been suggestions from recent work that it might be harmful, although this has not been conclusively proven. Thus, based on present knowledge I would not recommend selenium supplementation," says Burk, who studies the health effects of selenium.
How Much Selenium Do You Need?
Selenium is a naturally occurring trace mineral that is vital to good health. Low selenium has been linked to an increased risk of death and poor brain and immune function.
The government's recommended daily allowance for selenium is 55 micrograms for adults aged 19 and over. It is 60 micrograms daily for women who are pregnant and 70 micrograms for women who are breastfeeding.
Those levels aren't hard to reach. Thanks to selenium-rich soil throughout much of the country, most Americans get plenty of this essential mineral through meats and grains like corn and wheat.
"Your wheat that's used to make bread has quite a lot more selenium in it than ours would in Europe," Rayman says.
In fact, studies show the average selenium intake for men in the U.S. is about 134 micrograms per day. And that's a level that seems to be right on target for good overall health.
The U.S. Institute of Medicine has set a tolerable upper limit for selenium at 400 micrograms a day. Too much selenium can cause a condition called selenosis, which includes symptoms such as gastrointestinal upset, hair loss, white blotchy nails, garlic breath odor, fatigue, irritability, and mild nerve damage.
Selenium Supplements: Too Much of a Good Thing?
Hoping that more selenium might add up to even better health, researchers have tested supplements to see if they might boost immune function, brain health, and fertility, and ward off cancer and heart disease and stroke risk.
The review found that for people who have low selenium levels, taking supplements sometimes helps.
One study of adults in the U.K. who had low selenium levels, which are more common in Europe, found that people who took supplements were able to fight off a virus more quickly than those who took a placebo.
And supplements boosted sperm quality in men with fertility problems who also had low selenium intakes, allowing 11% to father a child. The men who took a placebo fathered no children. Selenium supplements have also shown promise for thyroid problems, though researchers say those results are early and need to be confirmed.
Studies in the U.S. that have tested supplements for cancer and heart disease protection have found no evidence of benefit, and indeed, in people who had the highest selenium levels going into the studies, taking supplements was tied to increased risks of harm.
Selenium and Diabetes
One study of more than 1,200 Americans, for example, found that those who took 200 micrograms of selenium daily for an average of nearly eight years had a higher risk of developing type 2 diabetes compared to those taking a placebo.
And those who started the study with the highest selenium levels -- 122 micrograms or higher -- saw a nearly three-fold jump in diabetes risk compared to those taking a placebo.
One limitation of that study, however, was that doctors didn't set out to study type 2 diabetes as an outcome. People were recruited to see if selenium could cut their risk for non-melanoma skin cancer.
Researchers concede that looking at outcomes that weren't part of the design of the study can muddy the results.
Still, other studies have also suggested an association between selenium and diabetes.
Having a higher selenium level was linked to an increased prevalence of diabetes in adults tracked by the CDC's National Health and Nutrition Examination Surveys.
In the same vein, a French study found that higher selenium levels were associated with having higher blood sugar levels.
Researchers note that selenium might have an effect on type 2 diabetes because at high levels, it can interfere with the body's ability to effectively use insulin.
When it comes to taking selenium, "It's horses for courses," says Rayman, using a British expression that means what's suitable for one person or situation might not be suitable for another.
"There wouldn't be a risk for us, in our population, if we took an extra 200 micrograms of selenium, but if you did that in North America, or in the U.S., then yes, you might well be putting yourself at risk," she says.
Second Opinion on Supplementing
"I think it's a balancing act. I think if people have a very strong, varied diet that's within the 2,000 calorie limit, there may be a case where you don't need the extra nutrients," says Duffy MacKay, ND, vice president of scientific and regulatory affairs for the Washington, D.C.-based Council for Responsible Nutrition.
"But in my clinical practice, you see a lot of people with a not-so-varied diet, or for one reason or another they're very limited in what they eat, and then the multivitamin does a nice job of filling those nutrient gaps," he says.
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