MINERAL METABOLISM 391 



Moreover, it was reportctl in 1957 that trace amounts of selenite ion 

 can replace vitamin E and cystine in ihc prevention ol degenerative 

 liver disease in rats, degenerative diseases ot liver, heart, kidney, and 

 muscle in mice, and an exudative disease of chicks. Since selenite 

 is more active on a weight basis than either of the organic compotinds, 

 the element may turn out to be essential in minute amounts. If, in- 

 stead, tocopherol in physiological quantities completely prevents the 

 development of symptoms, then selenium cannot be classed with the 

 completely essential elements. 



The physiologically active doses correspond to the levels of selenium 

 in normal tissues and would be supplied by all ordinary diets. The 

 actual requirements found experimentally are less than 1 per cent of 

 the toxic levels and amoimt to about 4 and 10 /xg. of selenimn per 

 100 g. of ration for rats and chicks, respectively. Approximately 2 to 

 5 mg. of tocopherol per 100 g. of feed is needed for similar protection. 

 Though work on the utilization of selenium by animals is new and 

 limited, there has been extensive and prolonged study of the toxicity 

 of this element. Field cases have been numerous in the semiarid 

 plains of the western United States with some reported in Alberta. 

 The only other troublesome area known is a small region in Ireland. 

 Symptoms of chronic poisoning appear when animals ingest plants 

 containing 3 to 4 p. p.m. of selenium for prolonged periods. Consump- 

 tion of plants with about 20 p. p.m. produces acute cases. 



Plants accumulate the element from the soil, depending on the 

 species and on the nature and pH of the soil. Large tracts of the 

 plains and mountain states are high enough in selenium compounds 

 to raise the vegetation to dangerous levels, including all the common 

 grain and forage plants. Worse still, certain species, mostly belong- 

 ing to a leginne genus Astragalus, concentrate selenium up to several 

 hundred to several thousand parts per million. These plants grow 

 where the selenium level of the soil is high, where plant growth tends 

 to be sparse because of limited rainfall, and where grazing is a logical 

 industry from the viewpoints of terrain, soil, weather, and available 

 moisture. At such levels acute poisoning is widespread, severe, and 

 almost unavoidable. 



Effective soil treatments have not been devised to remove the sele- 

 nium or prevent its uptake by plants. Obviously use of hay or grain 

 from such regions is quite dangerous, and extensive consumption of 

 cereals high in selenium could be a serious hazard to human beings. 

 However, vegetables take up but little of the element, milk contains 

 only traces, white flour is rather less dangerous than the whole grain, 

 and human diets as a whole seem bv accident to contain less than 



