FOODS 51 



3. The food must contain, not only the necessary amount of proxi- 

 mate principles, but these must be present in a digestible form. 

 As an instance of this, many vegetables (peas, beans, lentils) contain 

 even more protein than beef and mutton, but are not so nutritious, as 

 they are less digestible, much passing off in the faeces unused. 



The nutritive value of a diet depends mainly on the amount of 

 carbon and nitrogen it contains in a readily digestible form. A man 

 doing a moderate amount of work, and taking an ordinary diet, 

 will eliminate, chiefly from the lungs in the form of carbonic 

 acid, from 250 to 280 grammes of carbon per diem. During the 

 same time he will eliminate, chiefly in the form of urea in the 

 urine, about 15 to 18 grammes of nitrogen. These substances are 

 derived from the food, and from the metabolism of the tissues ; 

 various forms of energy, work and heat being the chief, are simul- 

 taneously liberated. During muscular exercise the output of 

 carbon greatly increases ; the increased excretion of nitrogen is not 

 nearly so marked. Taking, then, the state of moderate exercise, it is 

 necessary that the waste of the tissues should be replaced by fresh 

 material in the form of food ; and the proportion of carbon to nitrogen 

 should be the same as in the excretions : 250 to 15, or 16'6 to 1. 

 The proportion of carbon to nitrogen in protein is, however, 53 to 15, 

 or 3'5 to 1. The extra supply of carbon comes from non-nitrogenous 

 foods viz. fat and carbohydrate. 



Voit gives the following daily diet : 



Protein 118 grammes. 



Fat 100 



Carbohydrate 333 



Ranke's diet closely resembles Voit's ; it is 



Protein 100 grammes. 



Fat 100 



Carbohydrate 250 



In preparing diet tables, such adequate diets as those just given 

 should be borne in mind. The following dietary (from G. N. Stewart) 

 will be seen to be rather more liberal, but may be taken as fairly 

 typical of what is usually consumed by an adult man in the twenty- 

 four hours, doing an ordinary amount of work. 



