896 NUTRITION AND HEAT REGULATION. 



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

 formation of polypeptids p. 1010) . 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. The balance of the nitrogenous material is used, 

 in all probability, not for tissue maintenance or tissue growth, but 

 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 NH 2 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 

 serves for purposes of energy-supply, just as the non-nitrogenous 

 food-stuffs might do. We may suppose that it is the excess amino- 

 acids not used in tissue construction that undergo this fate, 



A third possibility in regard to the fate of the amino-acids, or rather the 

 absorbed products of protein digestion, has been discussed more or less at 

 times in physiology. In addition to the portion that is applied to the repair 

 or construction of tissue, it has been held that another portion may be syn- 

 thesized to a form of non-organized or non-living protein held as a sort of 

 storage supply in the liquids of the body, that is, the blood and lymph. It 

 has been supposed that this material is called upon first in fasting or starva- 

 tion, and that it constitutes an easily used form of protein food of limited 

 amount which is designated as circulating or storage protein. Our knowledge 

 in regard to this material is quite indefinite at present. Certainly none of the 

 known proteins of blood or lymph seem to discharge this function. 



It was supposed formerly that the process of deaminization 

 of the excess of the amino-bodies is accomplished mainly in the 



