Long term use of selenium food supplements may be linked to an increased risk of developing type 2 diabetes said researchers in a new US study.
The study is published in the journal Annals of Internal Medicine.
The discovery was a surprise to the scientists who were doing a follow up to studies of selenium in animals that showed it helped the body to handle glucose.
Instead they found that long term use of selenium supplements increased the risk of developing type 2 diabetes, a disorder often linked to obesity which occurs when the body cannot produce enough insulin to control blood sugar.
Selenium is a trace mineral that is found in the soil and becomes absorbed in the food chain. The body needs small amounts of it for healthy metabolism. It is an anti-oxidant and stops cells being damaged by oxygen.
Selenium is included in many multivitamin products and is also available as a food supplement on its own. Many people take it for their health, and some studies have suggested it can improve the way the body deals with sugar and prevent some of the problems of diabetes. Other studies suggest taking selenium supplements have no effect on diabetes or health.
This study involved 1,202 human patients who were already taking part in a clinical trial (Nutritional Prevention of Cancer trial, NPC) to assess the effect of selenium supplementation on skin cancer. None of them had diabetes when the study started. They were attending dermatology clinics in various parts of the eastern United States.
The scientists measured the patients’ blood selenium and randomly assigned them to take a daily dose of 200 micrograms of selenium supplement or a placebo.
The participants were monitored for 7.7 years on average. The incidence of diabetes in the two groups were then compared.
The results showed that 58 of the 600 people in the selenium group developed diabetes compared with 39 of the 602 in the placebo group.
The researchers calculated that the risk of developing type 2 diabetes over the 7.7 year period was about 50 per cent higher in the selenium group.
The authors said that:
“Selenium supplementation did not seem to confer benefit in terms of risk for type 2 diabetes in this sample of persons from low-selenium areas in the eastern United States. Instead, the cumulative incidence of type 2 diabetes was statistically significantly higher in the selenium group than in the placebo group.”
The results were independent of baseline age, sex, smoking status and body mass index (BMI) except for those in the highest 33 per cent of BMI.
More significantly, the risk for type 2 diabetes linked to selenium supplementation increased with greater blood levels of selenium at the start of the study.
The researchers concluded that these findings showed no overall benefit of selenium supplementation in preventing type 2 diabetes, conversely they suggested that:
“Long-term supplementation with 200 micrograms of selenium daily may adversely affect glucose metabolism.”
They said this is an important result for public health because selenium supplements in 30 to 200 microgram doses are used by people throughout the US and the western world.
However the authors said that despite these findings, there is evidence that selenium supplementation may help to prevent cancer. Larger randomized clinical trials are currently looking into this further.
The study also had limitations the researchers conceded. First, the main trial was not designed to look at diabetes but cancer. Secondly, the diabetes incidence was self-reported and there is a possibility that some patients may have been underdiagnosed for diabetes. Thirdly, although some other factors were taken into account such as age and BMI, there was no in depth analysis of family history, body fat distribution and physical activity, although these effects should have been minimized by the randomization.
And fourthly, the participant sample was not representative of the population at large, consisting of elderly people (mean age was 63.2 years) from low-selenium parts of the eastern US with a history of non-melanoma skin cancer. Thus these findings cannot really be generalized to other groups.
Writing in an accompanying editorial, Dr Eliseo Guallar, of Johns Hopkins University Bloomberg School of Public Health said that people should be getting enough selenium in their diet and there is no reason to take supplements. The results of this study don’t prove that selenium causes diabetes but they are worrying, said Guallar.
“Effects of Long-Term Selenium Supplementation on the Incidence of Type 2 Diabetes: A Randomized Trial.”
Saverio Stranges, James R. Marshall, Raj Natarajan, Richard P. Donahue, Maurizio Trevisan, Gerald F. Combs, Francesco P. Cappuccio, Antonio Ceriello, and Mary E. Reid.
Annals of Internal Medicine: 21 August 2007, Volume 147 Issue 4.
Written by: Catharine Paddock
Writer: Medical News Today