Similar risk reductions were also observed for people with high vitamin E levels, and amongst smokers. But the research, published in The American Journal of Clinical Nutrition appears to challenge the notion of risk reduction outside of these subgroups.
"Greater concentrations were associated with reduced prostate cancer risks in men who reported a high intake of vitamin E, in multivitamin users, and in smokers.," wrote lead author Ulrike Peters.
Previous studies have reported that the mineral is associated with reducing the risk of prostate and lung cancer, as well as boosting the immune system.
Over half a million new cases of prostate cancer are diagnosed every year world wide, and the cancer is the direct cause of over 200,000 deaths. More worryingly, the incidence of the disease is increasing with a rise of 1.7 per cent over 15 years.
However, results of the new nested case-control study, part of the Prostate, Lung, Colorectal, and Ovarian Cancer Screening Trial, suggest that increased blood levels of the mineral may not reduce the risk of prostate cancer in the wider population, but may offer a significant benefit for smokers, people with high vitamin E intake, and multivitamin users.
The researchers, from a range of universities and institutes in the US and Denmark, measured blood selenium levels 724 people with incident prostate cancer and 879 healthy controls.
After eight years of follow-up, Peters and co-workers report that, while no significant reduction in prostate cancer risk was found between serum selenium levels in the overall population, amongst those with high vitamin E intake (greater than 28 IU per day) high selenium blood levels were associated with a 42 per cent risk reduction, compared to those with low selenium levels.
Similar reductions in prostate cancer risk were observed in multivitamin users - for the highest compared with the lowest quartile of selenium blood levels a risk reduction of 39 per cent.
Moreover, higher serum selenium levels were associated with a 35 per cent reduced risk of prostate cancer amongst smokers, said the researchers.
The study has a number of limitations, particularly being based on case and controls, and dietary consumption of selenium containing foods or supplements may have changed on diagnosis of the cancer. Also, since the study was epidemiological no direct mechanistic study was conducted.
Significant further research is required, including more epidemiological studies and randomised controlled trials in humans, to further confirm these results, as well as investigating the underlying mechanism of how selenium appears to offer protection.
European selenium levels have been falling since the EU imposed levies on wheat imports from the US, where soil selenium levels are high. As a result, average intake of selenium in the UK has fallen from 60 to 34 micrograms per day, leading to calls from some to enrich soil and fertilizers with selenium to boost public consumption.
The European recommended daily intake (RDI) is 65 micrograms.
The researchers were from the University of Washington, Johns Hopkins University School of Medicine, the Washington University, the University of Colorado Health Sciences Center, The Danish University of Pharmaceutical Sciences, the Core Genotype Facility, and National Cancer Institute, National Institutes of Health.
Source: The American Journal of Clinical Nutrition January 2007, Volume 85, Number 1, Pages 209-217, "Serum selenium and risk of prostate cancer-a nested case-control study"
Authors: U. Peters, C.B Foster, N. Chatterjee, A. Schatzkin, D. Reding, G.L. Andriole, E.D. Crawford, S. Sturup, S.J. Chanock and R.B. Hayes