180 



BIOCHEMICALLY IMPORTANT MINERAL ELEMENTS 



the unit (micrograms) in which the iodine is expressed is only one-thou- 

 sandth as large as that used for the other elements. Cereals, as a class, 

 contain relatively high percentages of all these elements, but it must be 

 borne in mind that these high values are due largely to the high per- 

 centage of dry matter in cereals. Conversely, vegetables do not rank 

 as high on a percentage basis because of the large amount of water 

 contained in them. Perhaps a more correct basis for comparison would 

 be percentages of the foods as eaten, that is, in the cooked condition. 

 Differences resulting from water content would then be largely elimi- 

 nated. 



A second consideration that must be kept in mind is the quantity of 

 a particular food that is eaten. A food may be conspicuously high in 

 some element, but if it is eaten only occasionally, it contributes very little 

 toward the actual supply of that element in the diet. Lobster, for ex- 

 ample, is high in copper (1.5 mg. per 100 g.), but since lobster is eaten 

 rarely or not at all by the majority of people, it is a relatively unimportant 

 factor in supplying copper to the average diet. Another -example will 

 illustrate the converse situation. Although milk is low in iron, it is, be- 

 cause of the large quantity consumed, one of the largest contributors of 

 iron in the diet of a small child. 



On the basis of both composition and consumption, cereals and vege- 

 tables are the chief sources of supply of the trace elements. 



The mineral elements exist in living tissues partly in the form of in- 

 organic ions (K+, Na+, Ca+ + , Mg+ + , Fe+ + , C1-, SO4 — , HPO4 — , 

 H2P04~, etc.) dissolved in tissue fluids, and partly as components of 

 various organic molecules. Many of the latter are of particular bio- 

 logical importance and are therefore given special attention. In Table 

 8-2 are a few organic compounds that are known to exist in plant or 

 animal materials. 



Unusually large or small amounts of particular mineral elements occur 

 in the soil in various regions of the world, and this distribution often 

 causes the vegetation and drinking water in these areas to contain 

 correspondingly high or low amounts of the elements concerned. As a 

 result, domestic animals and human beings consume abnormal quantities, 

 frequently with serious, and even fatal, consequences. The best known 

 cases involve deficiencies of copper, cobalt, zinc, boron, and iodine in 

 some localities, and dangerously high concentrations of selenium, molyb- 

 denum and fluorine in others. These cases will be discussed below in 

 greater detail in connection with the individual elements. In general, 

 quantities of the mineral elements much larger than the physiologically 

 required amounts are likely to be toxic. This is particularly true of the 

 trace elements. 



