Selenium

In three large randomised placebo-controlled trials, selenium supplementation was given either alone [23] or with other elements [20,24].

Clark et al. [23] carried out a study in the USA on 1312 subjects to test whether selenium supplementation could reduce the incidence of non-melanoma skin cancer. Although no benefit was found for skin cancer, the group receiving the supplement had statistically significant reductions of approximately 40% and 50% in overall cancer incidence and cancer mortality, respectively. Based on these findings, there is a large randomised trial of selenium and prostate cancer prevention on-going.

The other two studies were conducted in Linxian, China. In the smaller study multivitamin supplements containing selenium was randomly assigned to 3.318 people with pre-existing oesophageal dysphasia. At the end of a 6-year intervention period the group receiving the supplement had statistically non-significant reduction of 7% in total mortality and 8% oesophageal/gastric cardia cancer mortality [24].

In the larger Linxian trial, 29.584 participants were assigned to receive four combinations of different nutrient supplements for 5 years. The group receiving the supplement with selenium, beta-carotene and vitamin A had a statistically significant reduction of 9% in all cause mortality and 13% in cancer mortality. In these two Chinese studies it is impossible to disentangle the effect of selenium from effects of other microelements [20].

It can be concluded that at present there is at most weak evidence that selenium supplements have value as cancer chemoprevention agents and they cannot be recommended for use in the general population in this context.