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How Much Selenium Do You Need? continued...

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.

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