People who take selenium supplements to prevent type 2 diabetes may increase their risk of developing it, according to news reports. An article in the Daily Mail on 11 July 2007 entitled "Selenium supplements could increase risk of diabetes" stated that it was "possible that some selenium compounds might themselves generate harmful free radicals capable of hampering the insulin-producing function of the pancreas" (11 July 2007).
Where did the story come from?
The study was conducted by Saverio Stranges and colleagues at the State University of New York at Buffalo and was published in the Annals of Internal Medicine .
What kind of scientific study was this?
This randomised, double blind, placebo-controlled trial recruited 1,312 people between 1983 and 1991 from dermatology clinics in the eastern United States for a study investigating whether selenium supplementation prevents cancer. When the study started 1,202 participants did not have diabetes, and it is data relating to these people that has been used for "secondary analysis" and informed Dr Stranges’s study.
Participants were chosen at random to receive either 200 micrograms per day of selenium or a placebo for over 7 years. During the study period, these people visited the clinic every six months for blood tests and to update the researchers about their general health.
What were the results of the study?
The authors found that there were 58 new cases of type 2 diabetes in the 600 people given selenium and 39 new cases of type 2 diabetes in the group given the inert tablets. The chance of developing diabetes was about 1.5 times higher in the group taking selenium supplementation compared with the group not taking it.
As this is a secondary analysis of the data, the possibility that the results are a chance finding is increased. The authors say they have carefully examined the data for other possible causes of this finding.
What interpretations did the researchers draw from these results?
The authors concluded that selenium supplementation does not seem to prevent type 2 diabetes and may increase the risk of developing the disease.
What does the NHS Knowledge Service make of this study?
The authors acknowledge some of the limitations of this study and have been cautious in their conclusions as it was based on a ’secondary analysis‘.
- Although no single study can provide the answer to a scientific question there are now a number of studies, which the authors mention, that show a similar effect.
- The mechanism of action of selenium is unknown, but as the data in this study were randomised there is a strong possibility that the effect of high-dose selenium is genuine. Before definitive advice can be offered regarding selenium intake, the mechanism of action needs to be further explored.
- Selenium is a common constituent of multivitamin pills taken by many people. These typically contain between 33 and 200 micrograms of selenium. As this intake is in addition to the selenium usually taken in food, it would seem sensible to take the authors’ advice and avoid high doses of selenium until more is known.
This study presents evidence of an association between taking selenium and the development of type 2 diabetes, but it is not clear if this is a causal relationship or not.
Sir Muir Gray adds...
There is no good evidence that selenium deficiency is common, as a mixed diet eaten in the UK does not appear to result in selenium deficiency.