Selenium, a trace element, could be an important cancer preventative, a major study has discovered. People given 200 micrograms a day reduced their chances of developing cancer by 39 per cent.

The trace element seems to be effective against certain cancers lung, prostate and colorectal (large intestine). It had no effect, for instance, on the skin cancers, basal and squamous cell carcinomas, for which the trial was originally created to test.

The research, prepared by the Arizona Cancer Center, is one of the first to provide such compelling evidence in favour of selenium. Earlier trials, especially one from China, were not so clear cut, although lower dosages of selenium were being used with these.

The Arizona trial involved 1312 patients, all with a history of skin cancers, who were treated for more than six years either with selenium or a placebo (the control group). The research was widened to include lung, prostate and colorectal cancers in 1990.

The group given selenium reported more cases of skin cancer than those given the placebo, but only 29 died from any cancer in the selenium group, compared with 57 in the control. The total incidence of cancer in the selenium group was 77, compared with 119 in the controls. So overwhelming was the effect that the researchers ended the trial early so the controls could start receiving selenium (JAMA, 1996; 276: 1957-63).

Selenium may also improve the body's defences against the effects of Aids, research has suggested. A group of HIV positive people were given either a placebo, beta carotene or 100 micrograms of selenium for one year. The selenium group showed the greatest increase in antioxidants, which help fight against Aids symptoms (Am J Clin Nutr, 1996; 64: 101-7).