368 PRACTICAL PHYSIOLOGY 



Fat. About 1 % fat has to be reckoned as contained in the meat prepared as 

 above. The rest is given best 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 substance 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 above scheme it will be seen that the 

 only food-stuff which contains nitrogen and sulphur is protein. We have, 

 therefore, two excretory products from the amount of which we can determine 

 protein metabolism. In the case of carbohydrates 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. 



I 



Proteins { -p , 



j^ Carbohydrates 



1. Protein. 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. 410). A 

 certain amount, however, appears in the faeces. With an ordinary diet most of 

 this latter comes from the unabsorbed protein, 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 protein free diet, and 

 for man it amounts on an average to 1 grm. per diem. During starvation it only 

 amounts to 0'2 grm., so that it is obvious that it comes from the digestive juices 

 and excreta poured into the intestine. 1 



The amount of nitrogen excreted in the sweat is so small as to be negligible. 

 The urinary nitrogen, plus one gramme per diem, as nitrogen excreted into the 

 intestine, gives us, therefore, the total amount of nitrogen excreted. Since 

 protein contains 16 % of nitrogen, each gramme of nitrogen corresponds to 6 '25 grm. 

 of protein, and since meat contains on an average 3 '4 % of nitrogen, each gramme 

 of the latter will correspond to 30 grm. of muscle. 



(6) From the amount of Sulphur Extracted. The proteins of food con- 

 tain 1 % of sulphur. This is excreted in the urine largely as sulphuric acid, and the 



1 In accurate metabolism determinations it is necessary to collect the faeces for 

 each day, to dry them slowly on a sand bath, and then to make the following 

 determinations : 



(a) The total amount of nitrogen. 



(b) The total amount of fat (i.e. extract with Soxhlet's apparatus). 



(c) The total amount of solids. 



