New research led by Dr. Saverio Stranges, Associate Professor of Cardiovascular Epidemiology at the University of Warwick’s Warwick Medical School has found that taking large selenium supplements may actually increase the risk of developing type 2 diabetes rather than protecting against it as previously thought.

Selenium is a trace mineral that is essential component of proteins involved in antioxidant activity. Experimental evidence from animal models and a few previous studies in humans suggest that selenium supplements may exert beneficial effects on glucose metabolism and delay complications of diabetes. However, before this University of Warwick study, no randomized, placebo-controlled clinical trials to date had tested the effect of long-term supplementation with selenium alone in the prevention of type 2 diabetes.

The researchers were able to use data from the Nutritional Prevention of Cancer (NPC) Trial, a double-blind, randomized clinical trial designed primarily to evaluate the efficacy of selenium supplementation for prevention of cancer in areas in the Eastern U.S. where selenium levels are lower than the national average. The selenium and diabetes study involved 1,202 people who did not have type 2 diabetes when they entered the cancer clinical trial. Participants had been recruited for the main study between 1983 and 1991, and they were involved for an average of 7.7 years. The supplementation study was completed in February 1996. Analysis for this diabetes study involved data from six hundred persons who had taken 200 micrograms of selenium and 602 who were randomized to receive placebo pills.

The results of their study were published online this week in the Annals of Internal medicine in a paper entitled Effects of Long-Term Selenium Supplementation on the Incidence of Type 2 Diabetes. Saverio Stranges, M.D., Ph.D., first author on this paper, now at the University of Warwick and previously at  State University of New York (SUNY). The researchers found that during an average follow-up of 7.7 years, type 2 diabetes developed in 58 selenium recipients and 39 placebo recipients. (an incidence, 12.6 per 1000 person-years versus 8.4 per 1000 person years for placebo). The lack of benefit of selenium supplementation on the incidence of type 2 diabetes persisted across subgroups of age, sex, body mass index, and smoking status. In addition, among participants taking selenium supplementation, those who had the highest levels of selenium in their circulation at the beginning of the study had the highest risk of developing type 2 diabetes over the average 7.7 years of follow-up.

The researchers’ conclusion is that Selenium supplementation does not appear to prevent type 2 diabetes, and it may increase risk for the disease. The findings are particularly important because selenium supplements in doses of 30 to 200 micrograms a day are widely used by the public in the United States and other Western countries. "At the moment we don’t know what mechanism or mechanisms account for this finding," said Stranges. "We have very little understanding on the possible biological pathways involved. In addition, our findings need to be replicated in larger clinical trials before conclusive evidence can be drawn on whether high doses of selenium supplements increase the risk of type 2 diabetes, as suggested by our study."

Additional authors are: Professor Cappuccio and Dr Ceriello from the University of Warwick’s Warwick Medical School, UK, James R. Marshal, Ph.D., Raj Natarajan, and Mary E. Reid, Ph.D from the Roswell Park Cancer Institute, Buffalo, NY; Richard P. Donahue, Ph.D., and Maurizio Trevisan, M.D. from the University at Buffalo; Gerald F. Combs, Ph.D.,from the Human Nutrition Research Center, Grand Forks, ND;.

Note for editors: Despite these findings, the researchers note that some evidence does still suggest that selenium supplementation may have a role in cancer prevention. Ongoing, large randomized clinical trials are addressing this question.

For further information contact:
Dr Saverio Stranges, MD PhD Associate Professor of Cardiovascular Epidemiology
Warwick Medical School, University of Warwick
Tel: + 44 (0) 2476968667
Email: S.Stranges@warwick.ac.uk

Kelly Parkes, Communications Officer, Warwick Medical School, University of Warwick.

Email: k.e.parkes@warwick.ac.ukTel: 0247 615 0483 or 07824 540863.