THE NORMAL DIET OF MAN 727 



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 action 

 of the vegetable diet on the movements of the alimentary canal, 

 so that the food is hurried through the intestine before the processes 

 of digestion and absorption have had time to attain their limit. This 

 last factor may interfere also with the digestion of animal protein 

 on a mixed diet. The fats and carbohydrates of the ordinary diet 

 of man are also utilised to a very large extent. The faeces passed 

 on a fat-free diet always contain between 3 and 6 grm. of ethereal 

 extract which is reckoned as fat. When the fat of the food consists 

 largely of olein and is fluid at the temperature of the body, it is 

 almost totally absorbed, the absorption becoming less as the melting- 

 point of the fat rises. Ordinary carbohydrates are also very well 

 absorbed, but here very large variations may be produced by altering 

 the condition in which they are presented in the food. The following 

 table shows the relative digestibility of the different food-stuffs in a 

 healthy individual on a normal diet : 



PERCENTAGE OF FOOD-STUFF ABSORBED 



Protein Fat Carbohydrate Ash Total energy 



Average of five j ^ 94 9? . 7 9Q>5 



experiments I 



In judging therefore of the sufficiency of any given dietary, 

 it is important to remember that on the average only about 90 per 

 cent, of the total energy of the food is available for use by the 

 body. If, for instance, the body requires an amount of energy 

 equivalent to 3000 calories per day, it would be necessary to give 

 food corresponding to 3333 calories per day. If, as is the case 

 with the poorer classes, the food consists mainly of vegetable 

 products it may be necessary to increase still further this allow- 

 ance, since on a diet such as rye bread the loss of energy in the 

 faeces may amount to as much as 35 per cent, of the total energy 

 of the food. 



The quantity of food which is necessary to keep an adult man 

 in a state of health, without loss or gain of weight, is represented 

 by that amount which is sufficient after absorption to supply the 

 total daily output of energy. This output will vary considerably not 

 only from individual to individual but also with the weight and 

 size of the man, and above all with his state of muscular activity. 

 Thus in the case of a woman weighing 49'45 kilos, in a state of 

 hysterical sleep, the total output of energy during twenty-four hours 

 amounted to 1228 calories, i.e. 24'8 calories per kilo body weight. 



