PHYSIOLOGICAL CHEMISTRY 369 



amount of this excreted bears a constant relationship to that of nitrogen, viz. 1 of 

 sulphuric acid for every 5 '2 gr. of nitrogen. Being less in amount, its deter- 

 mination is not so accurate as that of nitrogen, but it affords us a valuable control 

 in estimating protein metabolism, and is the only way by which we can estimate 

 this when nitrogenous bodies other than protein are contained in the diet. 



2. Fat and Carbohydrate. The end products of the metabolism of both 

 these bodies are water and carbon dioxide gas, and, of these two bodies, 

 the only one which it is possible to estimate with ease is the latter. 

 Protein, however, also contributes to the excretion of carbon dioxide, so that, 

 before we can know how much carbohydrate and fat are being oxidised in the 

 body, we must find out what proportion of the total carbon excreted is derived 

 from the metabolism of the protein. 



To estimate the total amount of carbon excreted, the expired air must be 

 collected, and a determination of the amount of carbon dioxide which it contains 

 made by one of the methods described (see p. 184). The obtained result 

 multiplied by 0'273 gives the amount of carbon excreted in the breath. A certain 

 amount of carbon is also excreted in the urine. This latter amount could be 

 directly determined by making an elementary analysis of the dried urine, but 

 such a method would, of course, be too laborious for metabolism work. In order 

 to determine this amount of carbon it is sufficiently accurate to multiply the 

 nitrogen excreted by 0'67, for it has been determined that, for every gramme of 

 nitrogen excreted, there is this amount of carbon, and that this ratio is a 

 constant one. 



Having estimated what the total excretion of carbon is, we must now ascertain 

 how much of it comes from protein. To do this multiply the total amount of 

 nitrogen excreted by 3'3 (since proteins contain approximately 52'8 of carbon, 

 and 16 of nitrogen). If this amount of carbon be deducted from the total amount 

 excreted, the remainder corresponds to the carbon derived from the combustion 

 of fat and carbohydrate. As to which of these two bodies it is from which the 

 carbon really comes, we have no means of telling definitely, except by a 

 determination of the respiratory quotient, but since there is very much more 

 fat than carbohydrate in the tissues we usually reckon it as fat. Each gramme 

 of carbon corresponds to T3 grammes of fat (because fat contains 76 '5 grammes 

 carbon). 



3. The Amount of Energy given out by the Body. The energy is 

 liberated in the body partly as heat, and partly as muscular work. The amount 

 actually lost as heat may be determined by placing the animal in a respiration 

 calorimeter, but it is difficult to estimate the amount lost as mechanical work. 



There are, however, certain indirect methods by which the total amount of 

 energy liberated may be determined, and these are as follows : (a) By comparing 

 the amount of food-stuffs taken in with the amount which reappears in the 

 excreta, we can find out how much of each food-stuff has actually undergone 

 metabolism in the tissues. It is now quite easy to find how much energy this 

 corresponds to, by multiplying the amount of each food-stuff metabolised by its 

 caloric value. (Where the diet contains both fat and carbohydrate, and where an 

 accurate balance of intake and output of carbon does not exist, we must reckon 

 the excess or deficit as fat, since there is much evidence to show that the amount 

 of carbohydrate in the body remains pretty constant. ) 



(6) The extent of oxidation in the tissues is determined, not by the amount of 

 oxygen inspired, but by the activity of the tissues. We can, therefore, employ 



2A 



