NUTRITION 353 



during fasting that equilibrium is established. Protein only 

 contains from 50 to 54 per cent, carbon, and the reason why 

 the animal under the above conditions goes on consuming more 

 and more protein until the balance is restored, is to enable the 

 required amount of carbon to be obtained. This fact explains 

 the extravagant nature of protein diets. It enables us to under- 

 stand the loss in body weight which may be occurring even when 

 nitrogenous equilibrium is established, and to grasp the funda- 

 mental principle in feeding — that carbohydrate or fat, contain- 

 ing 76-5 per cent, carbon, enables equilibrium to be established 

 on a lower plane in consequence of the economical effect they 

 exert on protein. In the case of a dog in nitrogenous equilibrium, 

 less than two-thirds the amount of protein will suffice to produce 

 this result if a small amount of fat or carbohydrate form part of 

 the diet. The economising effect of fat, starch, and sugar on 

 protein is one of the few well-established facts in dietetics. 



The History of Protein in the Body. — When describing the 

 digestion of protein, substances in the digestive canal, sufficient 

 was said to indicate the big gaps which exist in our knowledge 

 of this question. Still greater ignorance exists of the subject 

 now to be considered — viz., the behaviour and disposal of the 

 protein matters after they have entered the system through the 

 portal vein. It has been seen that under the combined action 

 of pepsin and trypsin the protein molecule is completely broken 

 up into a number of smaller molecules, and what may escape 

 the action of pepsin and trypsin is dealt with by erepsin before 

 entering the blood. The protein substances once in the blood 

 are utilised in the repair and restoration of the tissues, and a 

 study of nitrogenous metabolism has shown that under ordinary 

 circumstances the extent of this repair is probably not consider- 

 able, and it certainly has shown that whatever portion of the 

 protein is so used must turn out a similar amount of degraded 

 material from the tissues, or the outgo of nitrogen would be less 

 than the income. What is the probable behaviour of protein in 

 the body until the stage of urea is reached ? 



Pfliiger taught that the whole of the absorbed material must 

 first be converted into protein before any destruction of it can 

 occur ; in other words, that there is no short cut to urea excepting 

 through the disintegration of the living cell. Voit held that the 

 protein, when absorbed, is divided into two portions — one, the 

 smaller, repairs wear and tear in the body, and he spoke of it as 

 tissue protein; the other circulates with the blood and lymph, 

 and bathes the body cells, but does not form part of them. This 

 is destroyed by the tissues with the liberation of heat and the 

 formation of nitrogenous end-products, the chief of which is 

 urea. This portion Voit described as the circulating protein. 



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