FOODS 123 



THE DISPOSITION OF THE FOOD PRINCIPLES 



The Proteins. The protein principles of the food while in the ali- 

 mentary canal undergo a series of disintegrative changes by virtue of which 

 they are reduced in part to simple nitrogen-holding bodies, monoamino- 

 and diamino-acids and ammonia, and in part to their immediate antecedents 

 peptids and polypeptids, after which they are absorbed from the intestinal 

 contents. It has for some time been assumed that in the act of absorption, 

 the amino-acids and their immediate antecedents, are combined and trans- 

 formed into the form of protein characteristic of blood and tissue fluids, 

 viz., plasma albumin. This compound was looked upon as the immediate 

 source of the protein necessary for tissue growth and repair. The manner in 

 which the plasma albumin was transformed into the protein character- 

 istic of each tissue has never been satisfactorily determined. Recently 

 evidence has been adduced which makes it probable that the amino-acids 

 undergo no change in the act of absorption but enter the blood as such and 

 are carried direct to the tissues. On reaching any given tissue the cells absorb 

 and synthesize, perhaps under the influence of an enzyme, such amino-acids 

 as they may need for their growth and repair. The surplus amino-acids, 

 i.e., those not utilized in the synthesis of tissue protein, may be synthesized 

 to plasma-albumin, or stored unchanged or be deaminized, i.e., separated 

 perhaps by the action of an enzyme, into the amino-group, NH 2 , and some 

 carbonaceous radical. The amino-group is then combined with hydrogen, 

 and subsequently with carbon dioxid, to form ammonium carbonate which 

 is then transformed into urea, a transformation that takes place to some 

 extent in the muscles (Folin); the carbonaceous remainder may be trans- 

 formed into fat or sugar, which is subsequently oxidized thus contributing 

 to the production of heat. In the process of tissue metabolism the protein 

 molecule undergoes disintegration and gives rise to amino-acids, the different 

 elements of which may undergo changes similar to those just stated. The 

 ammonia absorbed from the intestine is changed to ammonium carbonate 

 carried direct to the liver and transformed into urea. 



The Fats. The fat principles while in the alimentary canal also undergo 

 a series of changes whereby they are reduced by enzymic action to soap and 

 glycerin, under which forms they are absorbed. During the act of absorp- 

 tion the soap and glycerin are synthesized to human fat. The fine particles 

 thus formed in the intestinal wall are carried by the lymph vessels to the 

 thoracic duct, and thence into the blood stream, from which they rapidly 

 disappear. Though it is possible that a portion of the fat enters directly 

 into the formation of the living material in general, it is generally believed 

 that it is at once oxidized and reduced to carbon dioxid and water with the 

 liberation of energy. The natural supposition that a portion of the synthe- 

 sized fat is directly stored up in the cells of the areolar connective tissue, thus 

 giving rise to adipose tissue, has been a subject of much controversy, though 

 modern experimentation renders this very probable. The body-fat, under 

 physiologic conditions, is mainly, however, a product of the transfomation 

 of carbo-hydrates. 



The question has again been raised as to whether the fine granules of 

 emulsified fat can be absorbed without undergoing this preliminary cleavage 

 into fat acids and glycerin. The evidence adduced in support of this view 

 is conflicting and for its settlement further experimental work is necessary. 



