SECTION VI 

 THE NORMAL DIET OF MAN 



THE adult man has to take a certain quantity of food every day 

 in order to furnish an amount of energy equal to that lost from the 

 body as heat and mechanical work, and to replace the waste of tissue. 

 This income is represented, not by the total food taken' into the 

 alimentary canal, but by the proportion of the food which is absorbed 

 from the canal. This will vary with the digestibility and nature of 

 the diet, and in any experiments instituted to determine the metabolism 

 of man the first question that must be decided is as to the proportion 

 of food-stuffs actually utilised. Food which is not absorbed will be 

 excreted from the body in the faeces. The degree of utilisation of 

 food-stuffs will therefore be given by an analysis of the faeces passed 

 daily on any given diet. In order to be certain that the faeces passed 

 during a given time correspond to and are derived from the food taken 

 in at the same time, it is usual to give at the beginning and end of the 

 experiment a capsule of lampblack, so as to colour the faeces and 

 delimit those formed during the period of the experiment. The faeces 

 during starvation contain a certain proportion of nitrogen, carbo- 

 hydrates, and fats, and in judging of the degree of utilisation of any 

 given food-stuff the amounts of these substances excreted by the 

 alimentary canal during starvation must be deducted from the total 

 amount found in the faeces. 



The excretion of nitrogen by the intestines varies between 0'54 

 and 1-36 grm. per day. If we find an amount of nitrogen in the 

 faeces not exceeding these figures, we are justified in concluding 

 that the utilisation of the nitrogenous constituents of the. food is 

 practically complete. This is the case when the man receives as 

 food the animal proteins, such as meat, eggs, or milk. In experiments 

 carried out with these materials the amount of nitrogen of the faeces 

 varied between O14 and 1-9 grm. Only when excessive amounts of 

 milk are given is the utilisation less complete and the nitrogen in 

 the faeces increased above this amount. If, however, the protein 

 be given in the form of vegetable food, the wastage of nitrogen by 

 the faeces is much greater. It may rise as high as 48 per cent, and 

 amount to 9 grm. per day. Nearly always it exceeds 15 per cent. 

 This greater wastage of the nitrogen of vegetable food depends partly 



726 



