CHAP, v.] NUTRITION. 833 



When an animal is fed simply on non-nitrogenous food, death soon 

 takes place ; the food rapidly ceases to be digested, and starvation 

 ensues. We can therefore only study the nutritive effects of 

 these substances when they form part of a diet which consists also 

 of at least some 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 con- 

 sumed. 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 carbohydrates 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 previously 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 507 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 grins, 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 may increase not only proteid 

 but also non-nitrogenous metabolism. This explains how an excess 

 of proteid food may, by the increase of general metabolism, 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 carbohydrates 

 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, 



