230 THE HUMAN MECHANISM 



secretin and other hormones; mucin; hemoglobin to realize 

 that the food must furnish material out of which to manu- 

 facture compounds of the greatest variety of chemical struc- 

 ture ; and for this purpose the greatest variety of material 

 must be furnished in the food. 



16. The unique position of protein. Considerations such as 

 the above give a glimpse into the unique value of protein 

 food. While all forms of carbohydrate yield the body, for 

 the greater part, only dextrose, and the fats yield only fatty 

 acids and soaps, all of them closely similar in structure, pro- 

 tein yields amino-acids of the greatest diversity of chemical 

 structure. The possibilities of chemical construction, or syn- 

 thesis (as it is generally called), are thereby greatly increased. 

 Only a chemically complex food like protein could serve for 

 the construction of the proteins of the living cell and for the 

 formation of the varied products of cell manufacture. Review 

 in this connection section 15 of Chapter VIII. 



Protein is also unique among the nutrients in the fact 

 that the body can make other nutrients from it. It is a well- 

 established fact that large quantities of sugar (dextrose) may 

 be made from protein, and we can therefore understand how 

 a dog living on fat and the leanest sort of meat (protein) 

 can do without carbohydrate in the diet. It is also possible 

 that at least small amounts of fat may be derived from pro- 

 tein through this intermediate stage of sugar, for fat may be 

 made from sugar. 



17. Variations in the nutritional value of individual pro- 

 teins. Until comparatively recent times all food proteins 

 were regarded as having equal value in nutrition, with the 

 single exception of gelatin, which has long been known to 

 be incapable of meeting the protein needs of the body. The 

 discovery that some food proteins are lacking in one or more 

 of the amino-acids, and that the same amino-acid may occur 

 in very small amounts in one protein and very large amounts 

 in another, suggested to two American physiologists, Mendel 



