174 FOODS [chap. 



blood, and are only re-formed into proteins by the cells 

 which remove them from the blood. The greater part 

 of the nitrogen compounds thus digested is oxidised 

 into carbonic acid and water, the nitrogen part of the 

 compound being eliminated from the blood by the 

 kidneys, chiefly as the urea which is excreted in the 

 urine. A part, however, of the digested protein — and 

 this is really the indispensable part to the animal — 

 is used up for repairing nitrogenous waste in the tissues. 

 In growing animals some of the proteins of the blood 

 are stored up as lean meat ; also, when there is an 

 excess of protein in the diet, some of the carbon will be 

 stored in the animal as fat, the nitrogen being again 

 excreted as urea. When an animal is not putting on 

 weight at all, the whole of the nitrogen that is digested 

 is excreted again as urea and kindred bodies in the 

 urine ; the amount of protein required to repair the 

 daily waste is comparatively small, and the excess is 

 simply treated by the animal as fuel. 



The fibre in the food analysis does not truly 

 represent the indigestible portions of the food, for 

 all natural foods except pure oil, sugar, and starch are 

 only imperfectly digested ; some part of the food resists 

 the intestinal enzymes and bacterial processes and is 

 excreted from the body in the faeces. As regards the 

 nitrogenous compounds of food, it is important to 

 remember that whatever is digested is afterwards 

 excreted as soluble urea in the urine, while the 

 undigested portions remain in an insoluble state in 

 the dung. The non-protein nitrogenous bodies pos- 

 sess a much smaller food value than the proteins; 

 as fuel they are not so valuable, while it is 

 still a matter of debate to what extent they can 

 replace proteins in repairing tissue waste. Appar- 

 ently they can at least save waste of the proteins, and in 



