METABOLISM, NUTRITION AND DIETETICS 471 



grammes of meat in the twenty-four hours to prevent his 

 body from losing carbon. For a man weighing 70 kilos, 

 the daily excretion of carbon on an ordinary diet is about 

 300 grammes. More than 2,000 grammes of lean meat 

 would be required to yield this quantity of carbon ; and, 

 even if such a mass could be digested and absorbed, more 

 than three times the necessary nitrogen would be thrown 

 upon the tissues. 



Not only may carbon equilibrium be maintained for a 

 short time in a dog on a diet containing fat only, or fat and 

 carbo-hydrates, but the expenditure of carbon may be less 

 than the income, and fat may be stored up. But, of 

 course, if this diet is continued, the animal ultimately dies 

 of nitrogen starvation. 



So far we have spoken only of the income and expenditure 

 of carbon and nitrogen ; and from these data alone it is 

 possible to deduce many important facts in metabolism, 

 since, knowing the elementary composition of proteids, fats 

 and carbo-hydrates, we can, on certain assumptions, translate 

 into terms of proteids or fat the gain or loss of an organism 

 in nitrogen and carbon, or in carbon alone. But the 

 hydrogen and oxygen contained in the solids and water of 

 the food, and the oxygen taken in by the lungs, are just as 

 important as the carbon and nitrogen ; it is just as neces- 

 sary to take account of them in drawing up a complete and 

 accurate balance-sheet of nutrition. Fortunately, however, 

 it is permissible to devote mu^h less time to them here, for 

 when we have determined the quantitative relations of the 

 absorption and excretion of the carbon and nitrogen, we 

 have also to a large extent determined those of the oxygen 

 and hydrogen. 



(3) Income and Expenditure of Oxygen and Hydrogen. The 

 oxygen absorbed as gas and in the solids of the food is 

 given off chiefly as carbon dioxide by the lungs ; to a small 

 extent as water by the lungs, kidneys, and skin ; and as 

 urea and other substances in the urine and faeces. The 

 hydrogen of the solids of the food is excreted in part as 

 urea, but in far larger amount as water. The hydrogen and 

 oxygen of the ingested water pass off as water, without, so 



