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Selenium: friend or foe?

A review of published evidence shows that selenium supplements may be helpful in preventing a range of diseases, but only in people who have low selenium levels. For those at higher levels, taking more could increase risk.

BMJ Group News

What do we know already?


Selenium is a nutrient that is incorporated into the body in proteins that have an effect on the immune system. The immune system fights off infection, and is also important in fighting cancer cells. Selenium is also used by the body in making sperm, and reducing inflammation. These various functions mean it could potentially affect health in many different ways.

Low levels of selenium in the body are associated with an increased chance of death in older people (although there are some problems with the studies that show this). Insufficient selenium is also associated with many conditions, including epilepsy, Alzheimer’s disease, male infertility, recurrent miscarriage, and thyroid disease. Low selenium has also been linked to prostate cancer.

However, studies looking at whether giving people extra selenium would improve their condition have had mixed results. In some cases, adding extra selenium seemed to increase the chances of certain types of cancer and of diabetes.

This may be because levels of selenium vary considerably, depending on where you live. The main sources of selenium are meat and grains, and typical content varies a lot depending on the type of soil. Soil in the US, for example, is much richer in selenium than much of the soil in Europe. That means most people living in the US have higher levels of selenium than Europeans. The benefits of adding extra selenium might disappear - or even reverse - after a certain level.

What does the new study say?

The new study reviews all the previous research in this area to draw some conclusions. The researcher says that, for people with generally low selenium levels (less than around 120 micrograms per litre of blood serum), supplementation is likely to be beneficial. Above that level, risks of cancer (especially squamous cell skin cancer) and diabetes seem to rise.

However, she cautions that the relationship between selenium and health is complex, and needs further research, especially in the area of diabetes.

How reliable are the findings?

This paper is a review of former studies, rather than new research. It brings together the conclusions of previous work, but it can only be as reliable as the original studies it includes.

What does this mean for me?

Most people in the UK have levels of selenium below 120 micrograms per litre of blood serum, according to previous studies.

This suggests that, for most people in the UK, selenium supplementation is likely to be safe, and may be helpful for those at risk of specific illnesses. It’s important not to take more than the recommended levels of supplements.

If you are concerned about your selenium levels, you could ask your GP whether it would be possible to have them measured, either by a blood test or by nail clipping analysis.

Published on February 29, 2012

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