Like with many other essential trace elements, the importance of selenium intake for the proper functioning of the human body (and the bodies of many other mammals, for that matter) has been discovered by studying specific illnesses caused by an almost complete lack of selenium in the diet. Not that such diseases would occur very often. They have been limited to certain areas of China where the soil shows an extraordinary lack of selenium. Primarily, if humans live on a diet that contains no selenium, or very low amounts of selenium, their immune systems will be impaired.

Another effect of a lack of selenium will only be noticed in the long range: as overall aging is very much related to free-radical damage, and as selenium has a function in protecting cells from free radical damage, those not taking care of a sufficient selenium intake may just age a little bit faster.

There have also been studies that showed that selenium had a clear function in the prevention of some cancers. Health guru Dr. Weil in early 1999 referred to a scientific study of 1312 participants from the Eastern coastal plains region of the US where selenium levels in the soil are low. Some of the participants received selenium supplements of 200 micrograms, while others where given placebos. The study was conducted for four-and-one-half years. Dr. Weil: "The people who had taken selenium had 63 percent fewer prostate cancers, 58 percent fewer colorectal cancers, and 46 percent fewer lung cancers than the rest of the group. Overall, there were 39 percent fewer new cancers among those taking selenium. And altogether, half as many died from their cancer."

Traditionally, humans get their selenium through plants they eat, with plants taking up selenium from the soil. If you live in an area where the soil contains little selenium, and if you eat only local vegetables and fruit, or if your dietary intake of selenium is low because you eat almost no vegetables and fruit, you may have an advantage of taking selenium supplements. Selenium is also one of the supplements, Dr. Weil takes himself. In his own words: " I will continue to take my 200 micrograms of selenium a day - the same dose used in the study - and suggest that you do too."

Those with a certain distrust towards synthetic forms of food supplementation can take care that their diets contain enough selenium by consuming about two brazil nuts a day. A substantial percentage of brazil nuts are indeed grown in Brazil, in a region with a soil high in selenium. The disadvantage in comparison to selenium in pill form: the selenium contents in brazil nuts is not as controllable, and theoretically, if your brazil nuts are grown not in Brazil but some place where the selenium contents in the soil are low, you may after all not get the selenium you bargained for.