DIET AND NUTRITION 



all changed by the organs of digestion so as to introduce 

 only glucose into the blood. We require at least ten in- 

 organic or mineral elements: sodium, potassium, calcium, 

 magnesium, chlorine, iodine, phosphorus, sulphur, iron, 

 and copper. Probably this list of indispensable inorganic 

 elements will eventually be extended to include several 

 others, such as manganese, zinc, silicon, fluorine, and 

 possibly nickel and cobalt, boron, etc. These last appear to 

 be necessary for the development of plants, and may also 

 be necessary for that of animals. At least six nutrient princi- 

 ples, called vitamins A, B, C, D, E, and G, are knovs^n to be 

 required for normal nutrition. Among the twenty or more 

 fatty acids known to chemists, at least twelve occur in 

 the fats of our ordinary foods, but it appears from the 

 results of experiment that only one of these, namely, lino- 

 leic acid, cannot be synthesized by the body from carbo- 

 hydrate molecules. From the data available we may say 

 with some assurance that the simplest diet which would 

 furnish everything necessary for normal nutrition must con- 

 tain appropriate proportions of at least thirty-six simple 

 chemical substances. Actually, we eat daily many times 

 this number in our ordinary foods of animal and vegetable 

 origin. 



Methods have been developed by means of which ex- 

 periments on animals fed a single natural food (wheat, 

 maize, etc.), which does not alone support satisfactory 

 nutrition, supplemented with single or multiple additions 

 of the indispensable nutrients, yield information concerning 

 the nature of their deficiencies. Such experiments may be 

 made nearly quantitative. Through such studies we have 

 secured a considerable body of knowledge concerning the 

 extent to which each of our more important natural foods — 

 cereal grains, tubers, fruits, roots, leaves, meats, milk, 

 eggs, etc. — furnishes the body with the indispensable 

 nutrients. In addition, such experiments have shown us 



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