876 STARCH AND SUGAR. [BOOK n. 



that the one seems to be a source of energy and the other not, 

 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 so 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 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 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, subsequently changed, during the act of 

 absorption, into dextrose, whereas cane sugar provides, in becoming 

 'inverted,' either while still in the alimentary canal or after 

 absorption, not only dextrose but the very different sugar laevulose. 

 Moreover if our laboratory experiments truly represent the digestion 

 taking place in the living body, the whole of the starch, 198, 

 is not changed into maltose, a part becoming some variety of 

 dextrine. Our knowledge of sugars and of their fate in the 

 economy is too imperfect for us to be able to state precisely 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 know- 

 ledge 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 



