THE CARBON BALANCE 207 



the lower figures applying to sedentary, the higher to 

 manual workers : — 



Carbohydrate . . . 370-570 grms. 



Fat 50-100 „ 



Protein .... 120-150 „ 



Since all three classes serve as a source of energy, the 

 question arises as to what extent each of these is necessary. 

 In view of the considerable powers possessed by the body 

 of converting one form into another, it might be thought 

 that each could to a large extent be replaced by either of 

 the others. 



The Corbohydrate Itequirement . — We have seen how the 

 body always maintains a constant sugar content of the 

 blood ; how when need arises, as in phloridzin diabetes, 

 it transfers proteins into carbohydrates. When carbo- 

 hydrates are withheld from the diet there follows a pro- 

 found disturbance of metabolism, due to incomplete 

 oxidation of fat. Carbohydrate, as such, is therefore a 

 necessary component of the diet, but the minimum amount 

 necessary is not known. 



The Fat Requirement. — When fats are absent from the 

 food, evidence of malnutrition soon appears, due, as we 

 now know, not to a lack of fat as fuel, but of certain 

 substances present in fat which act in some way other 

 than as energy producers. Whether the body can live 

 without fat as a source of energy is not yet determined. 



The contraction of isolated muscle can be carried to 

 the point of fatigue without any depletion of the fat 

 present in the muscle. But this may be due to the 

 absence of the circulation; there may be wanting some 

 hormone necessary for the preparation of the fat for the 

 furnace. 



The oxidation of fat occurs normally without inter- 

 mediate conversion into carbohydrate. This indicates that 

 in the hving cell carbohydrates and fats are being oxidised 

 together. 



