Chap, v.] NUTRITION. 661 



fill. But apart from this the two food-stuffs, fats and carbohy- 

 drates, must play different parts in the economy, so that the 

 one cannot be wholly substituted 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 physiological 

 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 dues 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 insig- 

 nificant that it may be neglected. Man however consumes 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 carbo- 

 hydrate of a diet were supplied as starch or as sugar. Our 

 knowledge of sugars and of their fate in the economy is too 

 imperfect for us to be able to state the effects on the body of 

 digested starch as compared with those of cane sugar or milk 

 sugar ; but that these are or may be different is shewn by the 

 experience of medical practice. In many cases the total effect 

 on the body of a diet from which cane sugar is as much as 

 possible eliminated, though starch be allowed, is very different 

 from that of one of which cane sugar forms an appreciable part. 



Concerning cellulose, which in herbivora appears certainly 

 to serve as a source of energy and to be a real food-stuff, our 

 knowledge will not allow us to decide whether it has any 

 special uses of its own, or whether the body is simply led to 

 utilize and make the best of what is a necessary accompaniment 

 of the starch of vegetable food. 



Concerning the salts present in a diet we need only repeat 

 what was said in § 420 that these, though affording of them- 

 selves little or no energy, are as essential a part of a diet as the 

 energy giving food-stuffs, in as much as they in some way or 

 other direct metabolism and the distribution of energy. And 



