STATISTICS OF NUTRITION 619 



with the supply of fat or fat-producing food, and there is by no 

 means the same prompt adjustment of expenditure to income in 

 the case of carbon as in the case of nitrogen. 



Carbon equilibrium can be obtained in a flesh-eating animal, like 

 a dog, with an exclusively protein diet; but a far higher minimum 

 is required than for nitrogenous equilibrium alone. Voit's dog 

 required at least 1,500 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 250 to 

 300 grammes. About 2,000 grammes of lean meat would be re- 

 quired 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 have to undergo preliminary cleavage and excretion 

 as urea or 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 proteins, fats, and carbo-hydrates, we 

 can, on certain assumptions, translate into terms of proteins pr 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 necessary to take 

 account of them in drawing up a complete and accurate balance- 

 sheet of nutrition. Fortunately, however, it is permissible to 

 devote much 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. 



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 far as 

 we know, undergoing any chemical change, or existing in any other 

 form within the body. But it is important to recognize that 

 although none of the water taken in as such is broken up, some 

 water is manufactured in the tissues by the oxidation of hydrogen. 



