METABOLISM, NUTRITION AND DIETETICS 545 



than one-third of the amount in Ranke's diet. It is obvious 

 that the endogenous protein katabolism sets the limit below 

 which it must be impossible permanently to reduce the allowance 

 of protein. But it would be very hazardous to assume that this 

 theoretical minimum limit corresponds with the permissible 

 physiological limit. From experiments on men of various callings 

 extending over many months, Chittenden has concluded that 

 the average man eats at least twice as much protein as he really 

 requires. We have already seen that the amount of nitrogen 

 required to repair the actual waste of the tissues is comparatively 

 small, and that with the ordinary amount of protein in the food 

 a very large fraction of the total nitrogen is rapidly excreted as 

 urea. There is no doubt, also, that many persons consume too 

 much protein, at any rate in the form of animal food, and would 

 feel better, work better, and probably live longer, if they 

 restricted themselves in this regard. But there is no evidence 

 that the digestion of such "quantities of protein as the average 

 healthy man eats, or the elaboration and excretion of the corre- 

 sponding amounts of urea, ' strain ' in the least the digestive 

 apparatus, the liver, or the kidneys. And it may just as well 

 be argued that it is advantageous that much more than the 

 minimum protein requirement should be offered to the tissues, 

 so that the appropriate amino-acids, even the scarcest of them, 

 may be sure to be present in sufficient amount, rather than that 

 the organs should be subjected to the unnecessary ' strain ' of 

 reconstructing some of the amino-acids themselves, supposing 

 that they possess this power. In a question of this sort the 

 immemorial experience and instinct of mankind cannot be 

 lightly waved aside. 



If we decide the matter merely on physiological grounds, we 

 may say that for a man of 70 kilos, doing fairly hard, but not 

 excessive, work, 15 grammes nitrogen and 250 grammes carbon 

 are a sufficient allowance. The 15 grammes nitrogen will be 

 contained in 95 grammes dry protein, which will also yield 

 50 grammes of the required carbon. The balance of 200 grammes 

 carbon could theoretically be supplied either in 450 grammes 

 starch or in 260 grammes fat. But it has been found by experi- 

 ment and by experience (which is indeed a very complex and 

 proverbially expensive form of experiment) that for civilized 

 man a mixture of these is necessary for health, although the 

 nomads of the Asian steppes, and the herdsmen of the Pampas, 

 are said to subsist for long periods on flesh alone, and a dog can 

 live very well on proteins and fat. The proportion of fat and 

 carbo-hydrates in a diet may, however, be varied within wide 

 limits. Probably no ' work ' diet should contain much less 

 than 40 grammes of fat, but twice this amount would be better ; 



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