324 HUMAN PHYSIOLOGY 



of a week or two, an animal has received 20 grams more 

 nitrogen than it has given back to its environment the 

 inference is that it has synthesized from the products 

 of digestion (20 X 6.25) or 125 grams of new protein. 

 If this were all represented by muscle the total would 

 be about 625 grams, for muscle is about 75 per cent, 

 water and contains perhaps 5 per cent, of other non- 

 protein matter, in other words, one-fifth of it is protein. 

 The increase in weight of a growing organism will ac- 

 cordingly be several times as great as the protein storage. 

 It must add water and mineral salts to make new tissue 

 and it is likely to add some fat. 



It will be apparent also that when nitrogen is lost 

 from the body, as in fasting, the weight must fall by 

 an amount many times greater than that of the nitrogen 

 excreted. The reasoning is parallel to that employed 

 above: a loss of 1 gram of nitrogen means a loss of 

 6.25 grams of protein, and this in its turn stands for the 

 destruction of perhaps 30 grams of average tissue. 

 A fasting animal will therefore excrete an amount of 

 water which cannot be accounted for on the basis of 

 income or oxidation; it is water set free by the dissolu- 

 tion of tissue. This is a fact often overlooked. 



The composition of the urine depends more upon the 

 protein intake than upon any other factor. -It will be 

 well to emphasize just here a negative statement, 

 namely, that the proportion and quantity of the urinary 

 constituents are but slightly influenced by muscular 

 activity. This is the same as saying that protein de- 

 composition is not materially increased by exercise. 

 The observation is an interesting one because muscle 

 is so largely made of protein that early writers naturally 

 assumed that this kind of material must be sacrificed in 

 the act of contraction. It has become clear that in our 

 consideration of muscle we must distinguish between 

 the machine and the fuel. The machine is constructed 

 chiefly of proteins (with water and salts), but the pre- 

 ferred fuel is sugar, with fat also available. 



