PHYSIOLOGICAL CHEMISTRY 367 



In the above table note particularly the great food value of such bodies as 

 bread, flour, cheese (particularly), lentils, peas, oatmeal, rice ; all comparatively 

 cheap articles of diet. 



The Methods for the Estimation of General Metabolism. Metabolism 



is the subject which treats of the changes undergone by the food-stuffs after they 

 are absorbed from the intestine. There are two subdivisions of the subject ; the 

 one, called general metabolism, has to do with the building up or breaking down 

 of the tissues. It derives its information from a comparison of the amount of the 

 various food-stuffs absorbed, with the amount of their excretory products. The 

 other, called special metabolism, has to do with the exact chemical changes which 

 absorbed food-stuffs undergo, and the localisation of the actual organ or organs in 

 which the various changes are effected. 



Space will only permit us to indicate some of the methods employed in study- 

 ing general metabolism, and to describe briefly how the results obtained may be 

 interpreted. The actual methods of analysis are fully described in other chapters, 

 and in the following description reference will be made to the pages on which the 

 most suitable method for each determination can be found. 



General Metabolism. In order to study this a balance sheet must be drawn 

 up, on one side of which is placed the intake (the amount of food and oxygen 

 absorbed), and on the other the output (the amount of the various bodies excreted 

 in the urine, faeces, breath, and sweat). 



I.* The Intake. The value of a diet can be expressed either as its chemical 

 value, or as its physical value. The chemical value means the amount of protein, 

 fat, carbohydrate, and salt which it contains. This is determined by referring 

 to analytical tables of the various food-stuffs (especially serviceable for this 

 purpose are the tables of Atwater). The amount of the various food-stuffs 

 administered can then be easily determined by multiplying the percentage given 

 on the tables \)y the amount of food given. When it is desired to be specially 

 accurate an actual analysis of the food is necessary, and when the metabolism of 

 protein is being specially studied, it is customary to determine the amount of 

 nitrogen which the food-stuff contains (Kjeldahl's method, p. 410), and this 

 multiplied by 6 '3 gives the amount of protein. 1 



The physiological heat value of a diet means the number of calories which it can 

 yield during its metabolism in the body. To" find the total heat value of the 

 diet, all that is necessary is to multiply the phj'siological heat values of the 

 administered food-stuffs by the amount of each which the diet contains. 



The Form in which the Food-Stuffs are best given for Metabolism 

 Experiments. Protein. This is usually given as meat, from which all the 

 visible fat and tendon are, as far as possible, removed. When calculating the 

 amount of protein from the nitrogen present, the gelatin and extractives which 

 the meat contains may be neglected, for gelatin, in the presence of an excess of 

 protein, has almost the same metabolic value as native proteins, and the 

 extractives exert no influence on the metabolism since they pass through the 

 tissues unchanged. Protein may also be administered as white of egg or as milk. 



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

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



1 This figure would not be correct if the food contained nitrogenous substances 

 other than protein. 



