ON DIET. 515 



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 labor 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 ( 455) seems possible or even prob- 

 able, 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 

 carbohydrate 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 learned 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 irrespec- 

 tive of each other. We are rather led to suppose that the accompaniment 

 of one set of changes, in some indirect manner, favors 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 mouth- 

 ful. 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 substi- 

 tuted for the other ; and though, beyond the fact that the one seems to be a 

 source of energy and the other not, we do not as yet know the true physio- 

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

 differently disposed hydrogen of the carbohydrate, we may perhaps infer 

 that the difference of use within the body of the two kinds of food-stuffs 

 bears not so much on their ultimate consumption to supply energy, as on 

 the various complicated processes which they undergo and arrangements in 

 which they take part before the end of their work is reached. We have 

 had a hint that the carbohydrate more rapidly supplies the heat-giving 

 metabolism than does the fat ; and this suggests an advantage to the economy 

 in receiving daily a certain portion of the more tardy material, while at the 

 same time it may be taken to mean that the fat before it is used to give rise 

 to energy has first to be converted into sugar, and so takes more time in its 

 work. 



The main carbohydrate of every diet is starch, and as far as we can learn 

 at present, the starch; which is so large a part of the cereals and vegetables 

 consumed by man is the same body in all of them ; for the use of such bodies 

 as inulin is so insignificant that it may be neglected. Man, however, con- 

 sumes no inconsiderable quantity of sugar, chiefly cane sugar. Since the 

 starch of a meal does not become available for the economy until it has been 

 converted into sugar, we might be inclined to infer that it was a matter of 

 indifference whether the carbohydrate of a diet were supplied as starch or 

 as sugar. But besides the fact that any large deficit of starch in a diet 

 might seriously interfere with the general course of digestion, especially if as 

 urged above the several digestive processes are more or less dependent on 

 each other, it must be remembered that the sugar into which starch is 

 changed by digestion is maltose, while cane sugar appears to be either 



