CH. XLI.] METABOLISM OF PROTEIN 621 



Metabolism of Protein. 



In our discussion of the origin of urea in the urine, we have 

 mentioned some of the main facts in relation to the metabolism 

 of proteins, and it would be well if the student again reads these 

 pages (pp. 586 to 588) before studying the paragraphs which now 

 follow; for the laws which govern the composition of urine are 

 the effect of more fundamental laws governing protein katabolism. 



Liebig was the first to divide foods into flesh-forming and heat- 

 forming, that is, into those which repair the tissue waste, and those 

 which are not so intimately assimilated into the protoplasm, but are 

 utilised as sources of energy. The latter function is the one per- 

 formed by the fats and carbohydrates, and the former is more 

 particularly the duty of the proteins. This idea is reflected in the 

 popular use of the term nutritious ; it is used almost synonymously 

 with nitrogenous, and the notion that the non-nitrogenous foods, 

 although they form the greater part of our daily diet, are not 

 nutritious and next door to useless, is a most mischievous one, 

 though it is carefully fostered by the advertisers of patent foods. 

 Both kinds of food are equally necessary, and equally though 

 differently nutritious. 



It is now known that the proteins are not only flesh-formers, 

 but also that they play the other role in nutrition and act as a source 

 of energy. The complete breakdown into amino-acids which occurs 

 in the gastro-intestinal tract, has in fact a double signification. It 

 enables the cells of the body to construct from the cleavage products 

 the proteins peculiar to themselves, and it further enables the body 

 to easily rid itself of the nitrogenous portions of the food-proteins 

 which are not wanted for the repair of tissue waste. This portion is 

 never really assimilated in the sense that it is built into protoplasm, 

 but it is taken in by the liver cells, which deamidise it and convert 

 the nitrogenous portion into urea. The non -nitrogenous moiety is 

 then utilisable for energy and heat production. 



In starvation, the income of the body is limited to oxygen, but 

 if water is given also, an animal will generally live a little over four 

 weeks. During this time the excretion of nitrogenous and carbona- 

 ceous waste continues and the body loses weight day by day. The 

 excretion of carbon dioxide continuously falls until death supervenes. 

 The nitrogen of the urine falls also within the first few days, and 

 then remains at a low but constant level to the end of the fourth 

 week. Then for the few days preceding death, its amount again 

 increases. By this date nearly every trace of the fat of the body 

 has disappeared, and so the cells fall back on their more precious 

 protein material and consume it in greater quantity than before. 

 The nitrogen elimination during the weeks when it remains constant 



