Clark published his seminal paper on selenium and cancer in 1996 (here). This paper was pivotal in nutritional research because it was the first large scale study to show the beneficial effects of selenium on cancer. Selenium supplementation of subjects in the trial caused a reduction of 50 % in cancer mortality and morbidity for all cancers. While these findings are of scientific importance their implications have not been widely emanated to the wider public arena. It is thought that selenium is able to decrease the risk of cancer because it is required as a cofactor in the enzyme glutathione peroxidase. This enzyme is pivotal in tissue defence to free radicals and as levels decline oxidative stress increases. It was Denham Harman who postulated back in the 1960’s that such free radical damage could initiate disease, which lead to the concept of the free radical theory of cancer. The ability of dietary selenium to increase tissue levels of glutathione peroxidase and to decrease oxidative stress supports the contention that the mineral may inhibit cancer.
In a follow up paper in 1998, Clark published further findings attesting to the efficacy of selenium supplements to inhibit cancer1. In the study, 974 men with a history of either skin cancer were administered either 200 μg per day of selenium or a placebo tablet. Treatment lasted for an average of 4.5 years and a follow-up a 6.5 years was performed to assess the effects of the treatment. The results showed that the selenium treatment was not associated with significant improvements of the primary end point of the incidence of skin cancer. However, selenium supplementation was associated with a 63 % reduction in the secondary endpoint of prostate cancer. In the placebo group 35 cases of prostate cancer were reported in the medical records of the subjects but the treatment group had only 13 cases. When the authors investigated on the subjects who initially had normal levels (<4 ng per mL) of the prostate specific antigen, there were only 4 cases of prostate cancer in the selenium group but 16 cases in the placebo group after 2 years.
In addition to the significant decrease in the incidence of prostate cancer, selenium supplementation was also associated with reductions in total cancer mortality and the incidence of lung and colorectal cancer. These results are therefore supportive of Clark’s pivotal 1996 paper showing significant reductions in cancer mortality and morbidity with 200 μg per day of selenium. The selenium tablets in this trial were a yeast form of selenium called SelenoPrecise that is a commercial product, and many similar products are widely available in health food stores. The probable reason that the selenium supplements had an anti-cancer effect is because the subjects were deficient in selenium. Studies from China show that populations living regions with selenium deficient soils have a significantly higher risk of cancer compared to similar populations living in regions with selenium rich soils. Further, supplementation with selenium reduces the incidence of cancer in these selenium deficient areas, but selenium rich food would likely have the same effect.
RdB
