476 PKACTICAL PHYSIOLOGY 



metabolism since they pass through the tissues unchanged. Proteid 

 may also be administered as white of egg or milk. 



Carbohydrate. That is best given as bread a day old, and always 

 obtained from the same source, so that its composition is constant. 



Fat. About 1 per cent, fat has to be reckoned as contained in the 

 meat prepared as above. The rest is best given as butter. 



When the investigation is being carried out on an animal, the whole 

 diet should be weighed out in the morning, after collecting the previous 

 day's excreta. It is seldom necessary to cook the food, but where 

 there is difficulty in persuading the animal to take some unpalatable 

 food substance, this latter may be mixed with the soup prepared from 

 the meat. When the experiments are being carried out on man, it 

 is of course necessary to cook the meat, and frequently also some of the 

 other food stuffs. The various constituents must be weighed out 

 before cooking, as it is impossible to know, after the food has been 

 prepared, the proportion of the substances used in cooking. For 

 observations extending over any length of time the diet should be 

 carefully chosen, and exactly the same amounts given each day. 



II. The Output. By referring to the following schema it will be 

 seen, that the only food stuff which contains nitrogen and sulphur is 

 proteid. We have, therefore, two excretory products from the amount 

 of which we can determine proteid metabolism. In the case of carbo- 

 hydrates and fats, on the other hand, there is no exclusive end product, 

 so that, in order to estimate the metabolism of these two bodies, it 

 is necessary to make a calculation. 



H 2 S0 4 



I N ^ 



Proteid s 



1. Proteid. The output of this is determined . 



(a) From the Amount of Nitrogen Excreted. Nearly the whole 

 of this occurs in the urine in which it is determined by Kjeldahl's 

 method (p. 243). A certain amount, however, appears in the faeces. 

 With an ordinary diet most of this latter comes from the unabsorbed 

 proteid, and must accordingly be deducted from the amount administered 

 in order to ascertain the actual amount absorbed. A certain amount 

 of it, however, comes from nitrogenous bodies, which are excreted into 

 the intestine from the blood. The actual amount of this excretory 

 nitrogen has been determined by feeding an animal with a proteid free 

 diet, and for man it amounts on an average to 1 gr. per diem. During 

 starvation it only amounts to 0*2 gr. so that it is obvious that it comes 



