Fish and shellfish contribute some selenium as well. The fact is, many well-nourished people rarely eat foods that are high in extractable selenium content. Selenium was recognized as an essential nutrient in the late 1920s. It also stipulates that intake should not exceed 400 micrograms, which is approximately twice the dose found in typical commercial supplements. Smoking and alcohol intake have been shown to be potent depressants of plasma selenium levels.
Selenium is quite inexpensive.
Though it's readily toxic in high doses, it has been shown again and again to be safe in what would likely be supplementational quantities. Selenium and Cancer Epidemiologic evidence for links between selenium and cancer has been accumulating for three decades. As of 1986, more than 100 such studies had been published and two out of three revealed a positive effect for selenium against cancer. Studies in the United States and Canada have shown that selenium soil content varies inversely with cancer incidence in both countries. There's also an inverse relationship between selenium blood levels and cancer rates.