CHAP, v.] NUTRITION. 875 



this we shall have to refer in speaking of a vegetable diet. 

 Against the statistical diet on the other hand we may urge that 

 instinct is not an unerring guide, and that the choice of a diet is 

 determined by many other circumstances than the physiological 

 value of the food. 



551. Taking however some such diet as the above to be the 

 approximately true normal diet, we may call attention to the fact 

 that the normal diet is made up of each of the three great food- 

 stuffs, carbohydrates being in excess. We may here remark inci- 

 dentally that the diets of both the carnivora and herbivora agree 

 with that of omnivora in containing all three food-stuffs : they 

 differ from each other as to the relative proportions only. As we 

 have seen, the body may be maintained in equilibrium on proteid 

 food alone; but an exclusively proteid diet is not only bought 

 dearly in the market, but also paid for dearly within the economy; 

 we are of course now speaking of man. To obtain the necessary 

 carbon out of the carbon moiety of proteid unnecessary labour is 

 thrown on the economy, and the system tends to become blocked 

 with the amides and other nitrogenous waste arising out of the 

 nitrogen moiety simply thrown off to secure the carbon. 



Fats and carbohydrates are much more akin to each other than 

 is either to proteid; and if on the one hand, as ( 542) seems possible 

 or even probable, the fat of the food and of the body is converted into 

 sugar either on its way to become built up into the tissue or in 

 the course of the changes taking place outside the real living 

 framework of the tissue by which it is reduced to carbonic acid, and 

 that on the other hand carbohydrates can furnish the fat whose 

 presence in the body is necessary, we might expect that carbo- 

 hydrate alone without fat might, with proteid, form a normal diet. 

 But on this point experience is probably to be trusted ; and we 

 may infer that in every normal diet some fat at least must be 

 added to the starches and the sugars. 



The advantage of this mixture is probably felt while the food 

 is as yet within the alimentary canal. What we have learnt 

 concerning digestion leads us to regard it as a complicated process, 

 and we cannot readily imagine that the proteolytic, amylolytic and 

 adipolytic changes run their several courses, especially in the small 

 and large intestine, apart from and irrespective of each other. We 

 are rather led to suppose that the accompaniment of one set 

 of changes, in some indirect manner, favours the others; and 

 it is for that reason probably that we take our food-stuffs not 

 separately but mixed in the same meal, often on the same plate 

 and even in the same mouthful. But apart from this the two 

 food-stuffs, fats and carbohydrates, must play different parts in the 

 economy, so that the one cannot be wholly substituted for the 

 other ; and though we do not as yet know the true physiological 

 function of the hydrogen of the fat as compared with that of the 

 differently disposed hydrogen of the carbohydrate, beyond the fact 



562 



