406 THE MILK OF MAMJIALS OTHER THAN THE COW. 



sugar can replace proteins, as these compounds form tlie only 

 source of nitrogen. Fat and sugar being composed of the same 

 three elements may replace each other, but it is evident that 

 in building up tissues containing high percentages of carbon 

 and hydrogen, fat is a far more advantageous food than sugar. 

 As a food for infants the value of milk largely depends on the 

 fat present, and it is doubtful whether fat can be replaced by 

 sugar without detriment to anabolic processes. 



As a food for adults, where the tissues are ready formed, milk 

 may be regarded chiefly as a source of energy. From this point 

 of view fat may be replaced by the iso-dynamic quantity of milk- 

 sugar. 



Heat of Combustion of Constituents. — The following 

 values for the heat of combustion of the constituents of milk are 

 due to Strohmer : — 



TABLE CXX\'. 



]?ats — Butter fat, . 9,231 -S calories per gramme. 



Other fats, . . 9,500 



Sugar — Milk-sugar, . . 3,950 „ „ 



Cane-sugar, . 3,955 ,, ,, 



Proteins — Casein, . . 5,858-3 „ ,, 



Albumin, . . 5,735-2 



These values assume that complete combustion takes place, 

 and that carbon dioxide, water, and nitrogen are produced. 

 In_the case of fat and sugar it may be fairly assumed that an 

 approach to complete combustion takes place in the human 

 body, and that carbon dioxide and water are excreted. The 

 nitrogen of proteins is not excreted as nitrogen, but as com- 

 pounds, of which urea may be taken as the type. 



Strohmer calculates that 1 gramme of average protein yields 

 0'3428 gramme of urea, the heat of combustion of which is 

 2,537 calories per gramme ; the heat of combustion of the urea 

 from 1 gramme of proteins is, therefore, 869 "7 calories, or, in 

 round figures, 15 per cent, of the total heat of combustion. It 

 is necessary, therefore, to deduct 15 per cent, of the heat of 

 combustion of proteins in calculating iso-dynamic metabolic 

 ratios. 



In round figures, the following will be the calories per gramme 

 developed in combustion of the three constituents in the" human 

 body :— 



Calories. 

 Fat, . 9,230 



Sugar, . . . 3,950 



Proteins, . . .4 970 



These figures are in the ratio of 2 "38 : I : 1 •26. 



