360 ESSENTIALS OF PHYSIOLOGY. 



the latter to uric acid. Uric acid is thus the end product of the 

 action of these enzymes on nucleic acid. The whole of the uric acid 

 formed in this way is not excreted as such in the urine, since many 

 tissues, more especially the liver, contain uricolytic enzymes, which 

 break down uric acid, one of the products being urea. 



The amount of uric acid appearing in the urine does not necessarily 

 represent the whole of that formed from the nucleo-proteins of the 

 food, but is derived partly from them, forming exogenous uric acid, 

 and partly from the breaking down of the nucleiris in the tissues, 

 endogenous uric acid. It will be seen from the table on p. 357 that, as 

 a rule, half the uric acid in the urine is of endogenous and half is of 

 exogenous origin. 



When the diet is free from nucleo-protein, the excretion of endo- 

 genous uric acid is extremely constant, but it is increased after severe 

 muscular exercise and also in fever, owing to a greater breaking down 

 of the nuclei in the cells of the body. 



The exogenous fraction varies in amount with the character of the 

 diet, being absent when this contains no nucleo-protein, and increased by 

 food such as kidney, sweetbread, and meat, which are rich either in 

 nucleo-protein or in the precursors of uric acid, such as hypoxanthine. 



Nucleo-proteins are not only broken down, but can also be 

 synthesised in the body. In the growing infant, for example, nucleo- 

 protein is rapidly being laid down in the body, although the food 

 (milk) contains hardly any nucleo-protein. 



SECTION V. 



The separate consideration of the changes undergone by fat, protein, 

 and carbohydrate, though convenient, represents very imperfectly the 

 complex nature of the metabolic changes in the body as a whole. 

 These are greatly influenced both by the nature of the diet and by the 

 absence of one or more of the food-stuffs from the diet. 



Starvation. The metabolism during starvation has been studied in 

 professional fasting men and also in the lower animals. When food is 

 withheld the store of glycogen in the body is rapidly used up, and after 

 two or three days the animal derives its energy solely from fat and 

 protein. The metabolism as a whole is diminished, and the consumption 

 of protein is reduced as far as possible, most of the energy needed by 

 the body being obtained by the oxidation of the fat previously present 

 in the fat depots. After three or four days the output of nitrogen in 

 the urine reaches a low level, which continues until the body fat 

 has been used up, and the sole source of energy is the tissue protein. 



