894 NUTRITION AND HEAT REGULATION. 



kind of protein are combined in general after the manner of the 

 formation of polypeptids (p. 1020). This portion of the nitrogenous 

 material that is used to replace tissue waste or to provide new tissue 

 has long been designated as tissue protein, and in the adult ani- 

 mal a relatively small fraction of the nitrogenous food may suffice 

 for this purpose. Considering the varying composition of proteins 

 in amino-acids it is probable that in this matter of replacing tissue 

 or providing material for new tissue some protein food-stuffs 

 may be more effective than others, since they may contain more 

 nearly the particular proportions of amino-acids essential in the 

 construction of human tissue protein. Experiments seem to bear 

 out this a priori probabihty. Thomas,* in experiments upon him- 

 self, found in general that the proteins furnished by animal foods 

 are more efficient in this respect than those obtained from vege- 

 table foods. Designating the efficiency of cows' milk as 100, he 

 finds that ox meat has a value of 104; fish, 95; crab meat, 79; 

 peas, 56; wheat flour, 40; cornmeal, 30. These numbers represent 

 what he calls the biologic value of the several sources of protein 

 commonly used in our diets The amino-acids not employed in 

 the construction of tissue protein are utilized presumably as a 

 source of energy. When the protein food is large in amount 

 the greater portion no doubt fulfils this function. When the 

 protein food is reduced to minimal proportions the amount left for 

 energy purposes is diminished correspondingly. It will be shown 

 later that ordinarily we eat more protein daily than is required to 

 cover our tissue wastes, that is, to provide the necessary tissue- 

 protein, so that on a customary diet it is probable that a good pro- 

 portion of the products of protein-digestion is used to furnish energy 

 rather than to construct tissue. The further history of the por- 

 tion used for energy purposes is not wholly known, but the main 

 point is that the amino-acids constituting this portion undergo 

 deaminization. The NH2 group is split off to form ammonia that 

 is subsequently converted to urea and excreted, while the organic 

 acid radicle which is left is oxidized to furnish energy or is built into 

 sugar or fat and oxidized at some later period. This portion of the 

 amino-acid, therefore, may be regarded as a source of energy 

 entirely equivalent to that furnished by the non-protein elements 

 of the food, the carbohydrates and the fats. If we grasp the funda- 

 mental idea that from the supply of amino-acids furnished to the 

 tissues by the digested food-proteins certain ones are selected to 

 construct the peculiar tissue-protein of the animal, so far as this is 

 required for growth or tissue-repair, then it is evident that the 

 balance is not needed as nitrogenous material, and its nitrogen is 

 removed by deaminization, while the remainder of the molecule 



* Quoted from Lusk, "Science of Nutrition," 3d ed., 1917, p. 371. 



