TUESDAY, Feb. 28 (HealthDay News) -- While getting the right amount of selenium in your diet can boost your immune function and lower your risk of death, you can get too much of a good thing. Higher-than-normal levels of selenium may contribute to the development of type 2 diabetes, hair loss and certain cancers, a new review of evidence finds.

"There is a U-shaped relationship between selenium intake and health. As selenium intake goes up from a low value, health improves until the bottom of the U-shaped curve is reached, but then adverse -- or even toxic -- effects begin to be seen," said Margaret Rayman, the author of the review of selenium research, and a professor of nutritional medicine at the University of Surrey, in England.

Selenium is a mineral found in a variety of foods. The amount of selenium in foods depends largely on where you live, as the selenium content in the soil varies. Selenium enters the food chain through plants, Rayman said. And, when animals consume the plants, they also consume selenium. Common sources of selenium include Brazil nuts, fish, poultry and wheat. Selenium is also available in supplement form.

Intake of selenium is high in Venezuela, Canada, the United States and Japan, according to background information in Rayman's review, while it's lower in Europe and some areas of China.

The average daily intake recommendations for selenium are 60 micrograms per day for men and 53 micrograms per day for women, according to the research.

For the study, Rayman searched medical literature to find previously completed studies on selenium. The results of that review appear online Feb. 29 in The Lancet.

Rayman found that daily intake of selenium varied from as little as 7 micrograms per day to as much as 4,990 micrograms per day. In Europe, the average intake was 40 micrograms per day, and in the United States, the average daily intake was 93 micrograms for women and 134 micrograms for men. Selenium supplements are likely part of this intake, Rayman said. That may be especially true in the United States where about half the population takes dietary supplements. Selenium is often found in multivitamins.

Rayman found several studies linking low selenium intake to a higher risk of dying from all causes as well as from cancer.

There's also some evidence that selenium levels can affect immune system function. Rayman found studies that suggest that selenium supplementation decreased hospital admissions due to infection for people who have HIV.

Selenium also plays an important role in brain function, according to the review. In a study of adults older than 65, performance assessments of coordination were worse in people who had low selenium levels. There was also an increased incidence of Parkinson's disease in people with low selenium. Too little selenium may also increase the risk of dementia, the review found.

"Low selenium status has been associated with higher risk of mortality, poorer immune function and cognitive [brain] decline," Rayman said. "Increasing selenium intake can help our ability to handle viruses, increase successful male and female reproduction, and reduce the risk of autoimmune thyroid disease. There is also some evidence that selenium may reduce the risk of cancer."

But, higher levels of selenium don't come without risk. People with the highest levels of selenium intake may have a greater risk of type 2 diabetes, non-melanoma skin cancers, hair loss and skin rashes, according to Rayman.

A supplement industry spokesman weighed in on the findings.

"There are many established benefits of selenium, and if you don't get adequate intake, you may be forgoing those benefits. There's a small amount of evidence that too much of anything may have a risk, but there's a U-shaped curve, which means with too little, there are clear risks," said Duffy MacKay, vice president of scientific and regulatory affairs for the Council for Responsible Nutrition.

MacKay noted that Americans likely have normal or higher levels of selenium because they take supplements. "I wouldn't want these results to be interpreted to mean that people should stop taking supplements," he said.

So, how can you be sure you're getting adequate levels without getting too much? One way is to get a blood test to assess your current selenium levels.

However, if you don't get a blood test, Rayman said, "One can be fairly confident that in North America, additional selenium isn't needed, but the same may well not be true in Europe."

Rayman said that people generally don't need to be concerned about the amount of selenium in their diets. The only food that might provide higher-than-recommended amounts when consumed frequently is Brazil nuts, she said.

More information

Learn more about selenium from the U.S. National Library of Medicine.