608 METABOLISM, NUTRITION AND DIETETICS 



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. 

 We have already considered (p. 240) the gaseous exchange in the 

 lungs, and we have seen that all the oxygen taken in does not 

 reappear as carbon dioxide. It was stated there that the missing 

 oxygen goes to oxidize other elements than carbon, and especially 

 to oxidize hydrogen. We have now to explain more fully the cause 

 of this oxygen deficit 



The Oxygen Deficit. The carbo-hydrates contain in themselves 

 enough oxygen to form water with all their hydrogen ; they account for 

 a part of the water-formation in the body, but for none of the oxygen 

 deficit. 



The fats are very different; their hydrogen can be nothing like com- 

 pletely oxidized by their oxygen. A gramme of hydrogen is contained 

 in 8'5 grammes of dry fat, and needs 8 grammes of oxygen for its com- 

 plete combustion. Only I gramme of oxygen is yielded by the fat 

 itself; so that if a man uses 100 grammes of fat in twenty-four hours, 

 rather more than 80 grammes of the oxygen taken in must go to 

 oxidize the hydrogen of the fat. 



The proteins also contribute to the deficit. In 100 grammes of 

 dry proteins there are 15 grammes of nitrogen, 7 grammes of hydrogen, 

 and 21 grammes of oxygen. The carbon does not concern us at present. 

 The 33 grammes of urea, corresponding to 100 grammes of protein, 

 contains 15 grammes of nitrogen, a little more than 2 grammes of 

 hydrogen, and a little less than 9 grammes of oxygen. There remain 

 5 grammes of hydrogen and 12 grammes of oxygen. But 5 grammes of 

 hydrogen needs for complete combustion 40 grammes of oxygen ; there- 

 fore 28 grammes of the oxygen taken in must go to oxidize the hydrogen 

 of 100 grammes of protein. Taking 140 grammes of protein as the 

 amount in a liberal diet for a man, we get 39 grammes as the required 

 quantity of oxygen. This, added to the 80 grammes needed for the 

 hydrogen of the fat, makes a total of, say, 120 grammes, equivalent to 

 about 85 litres of oxygen. A small amount of oxygen also goes to 

 oxidize the sulphur of proteins. 



With a diet containing less fat and protein and more carbo-hydrate, 

 the oxygen deficit would of course be less. 



The Production of Water in the Body. One gramme of hydrogen 

 corresponds to 9 grammes of water. In 140 grammes of proteins and 

 100 grammes of fat there are, in round numbers, 22 grammes of hydro- 

 gen; in 350 grammes of starch, 21-5 grammes. With this diet, 

 43*5 grammes of hydrogen is oxidized to water within the body in 

 twenty-four hours, corresponding to a water production of 391 grammes, 

 or 15 to 20 per cent, of the whole excretion of water. It has been 

 observed that during starvation the tissues sometimes become richer 

 in water, even when none is drunk. The only explanation is that the 

 elimination of water does not keep pace with the rate at which it is 

 produced from the hydrogen of the broken-down tissue -substances, or 

 set free from the solids with which it is (physically ?) united. 



Inorganic Salts. The inorganic salts of the excreta, like the 

 water, are for the most part derived from the salts of the food, 



