HANDBOOK OF NUTRITION: IX

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Abstract
The many mineral elements which exist in animal tissues occur in widely varying amounts. They range from calcium, which comprises approximately 2 per cent of the adult human body weight and which can be expressed in kilograms, down to those which we must measure in milligrams and even micrograms, and which have been termed "trace elements." The dividing line between trace and non-trace elements is purely arbitrary and a matter of choice. Some nutritionists1 include in the former category any element occurring in the tissues or nutritionally necessary in amounts equal to and less than iron, but others2 consider only those elements below iron. Presaging the point of view that traces of minerals might exert profound and specialized physiologic effects were the discoveries that iodine occurs in the thyroid,3 copper in octopus blood4 and in the hemocyanin of crustacea,5 zinc in the hemosycotopin of oysters,6