CHAPTER VII 

 METABOLISM, NUTRITION AND DIETETICS 



WE return now to the products of digestion as they are absorbed 

 from the alimentary canal, and, still assuming a typical diet 

 containing proteins, carbo-hydrates and fats, we have to ask, 

 What is the fate of each of these classes of proximate principles 

 in the body ? what does each contribute to the ensemble of vital 

 activity ? It will be best, first of all, to give to these questions 

 what roughly qualitative answer is possible, then to look at 

 metabolism in its quantitative relations, and lastly to focus 

 our information upon some of the practical problems of dietetics, 

 i. Metabolism of Proteins. The two chief proteins of the 

 blood-plasma, serum-globulin and serum-albumin, must, as has 

 been already pointed out, be recruited from proteins absorbed 

 from the intestine and for the most part, at any rate, profoundly 

 altered in its lumen and in their passage through the epithelium 

 which lines it. The physiological reasons for this alteration are 

 in a measure known, and have already been alluded to in con- 

 nection with the digestion of proteins. No doubt the far-reaching 

 decomposition of the protein molecule may to some extent 

 facilitate the absorption of protein food. No doubt also it is 

 imperative that such slightly hydrolysed products as peptone, 

 and particularly proteose, should not appear in quantity in the 

 blood, for when injected they cause profound changes in that 

 liquid, one expression of which is the loss of its power of coagula- 

 tion, and are rapidly excreted by the kidneys, or separated out 

 into the lymph. But the passage of the food from the stomach 

 is so gradual an affair, the quantity of digesting protein present 

 at one time in any loop of intestine is so small, and the rush of 

 blood which irrigates the active mucosa is so large, that the con- 

 centration of peptone or proteose necessary to produce injurious 

 effects could hardly in any case be realized. Again, there is no 

 evidence that the simpler decomposition products of further 

 hydrolysis are not in equal concentration as poisonous as proteose 

 and peptone. 



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