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 repre- 

 sented, not by the total food taken into the alimentary canal, but by the pro- 

 portion 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 utilisa- 

 tion 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, carbohydrates, and fats, and in judging oi 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 

 grai. 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 nitro- 

 genous 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 nitro- 

 gen of the faeces varied between 0-14 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 on the fact that certain non-protein and 

 indigestible nitrogenous constituents of seeds and grains are reckoned as 

 protein, partly on the fact that a considerable proportion of the protein may 

 be enclosed in indigestible envelopes, and partly on the greater stimulant 



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