628 GELATIN AS FOOD. [Book ii. 



fore only study the nutritive effects of these substances when 

 they are taken together with proteid material. 



When a small quantity of fat is taken, in company with a 

 fixed moderate quantity of proteid material, the whole of the 

 carbon of the food reappears in the egesta. No fat is stored 

 up ; some even of the previously existing fat of the body may 

 be consumed. As the fat of the meal is increased, a point is 

 soon reached at which carbon is retained in the body as fat. 

 So also with starch or sugar ; when the quantity of this is 

 small, there is no retention of carbon ; as soon however as it 

 is increased beyond a certain limit, carbon is stored up in the 

 form of fat or, to a smaller extent, as glycogen. Fats and car- 

 bohydrates therefore differ markedly from proteid food in that 

 they are not so distinctly provocative of metabolism. This is 

 exceedingly well shewn in the results obtained on the pig pre- 

 viously mentioned. It was found that 472 units of fat were 

 laid on for every 100 units of fat taken as such in the food 

 (which consisting of barley-meal, &c. contained a very small 

 amount of actual fat), while for every 100 units of the total 

 dry non-nitrogenous food including fat, starch, cellulose, &c, 

 no less than 21 units were retained in the body in the form of 

 fat. No clearer proof than this could be afforded that fat is 

 formed in the body out of something which is not fat. In 

 § 401 we have already discussed this formation of fat out of 

 carbohydrates. 



As one might imagine, the presence of fat or carbohydrates 

 in the food is found to decrease the amount of proteid material 

 necessary to establish nitrogenous equilibrium. For instance, 

 with a diet of 800 grms. meat and 160 grms. fat, the nitrogen 

 in the egesta became equal to that in the ingesta in a dog, in 

 whose case 1800 grms. meat had to be given to produce the 

 same result in the absence of fats or carbohydrates. 



On the other hand, it was found that, with a fixed quantity 

 of fatty or carbohydrate food, an increase of the accompanying 

 proteid led not to a storing up of the surplus carbon contained 

 in the extra quantity of proteid, but to an increase in the con- 

 sumption of carbon. Proteid food increases not only proteid 

 but also non-nitrogenous metabolism. This explains how an 

 excess of proteid food may, by the increase of general metabo- 

 lism, actually reduce the fat of the, body. 



We have at present no exact information concerning the 

 nutritive differences between fats and carbohydrates, beyond 

 the fact that in the final combustion of the two, while carbo- 

 hydrates require sufficient oxygen to combine with their carbon 

 only, there being already sufficient oxygen in the carbohydrate 

 itself to form water with the hydrogen present, fats require in 

 addition oxygen to combine with some of their hydrogen. 

 Hence in herbivora, living largely on carbohydrates, a larger 



