518 NUTRITION. 



give rise to lactic acid. Indeed, when we speak of the digestibility or the 

 indigestibility of this or that article of food, we do not in many cases so 

 much mean the relative amount of the substance taken up in some way or 

 other by the alimentary canal as the characters advantageous or otherwise 

 of the changes which it undergoes in being so taken up. 



Hence the purely chemical statement of the amount of potential energy 

 present in an article of food is no safe guide of the physiological value of 

 the substance. A chunk of cheese stands very high on, generally at the 

 top of, a table of the nutritive value of articles of food drawn up on ex- 

 clusively chemical principles according to the units of energy present in 

 a unit of the material but it is very low down in a corresponding physio- 

 logical table. And similarly a dish of old peas has a very different physio- 

 logical function from a plate of fresh meat, even when both contain the 

 same amount of nitrogen. 



In thus correcting for digestion the nutritive value of a diet it must 

 also be borne in mind that the alimentary canal, while chiefly a receptive 

 organ, is also to some extent ( 245) an excretory organ ; a free passage 

 through the canal is needed not only for carrying off undigested matter but 

 also for getting rid of excreted matter ; and the presence of the former, up 

 to certain limits, assists the discharge of the latter. Were it possible to 

 prepare a diet every jot and tittle of which could be digested and absorbed, 

 the use of such a diet would probably bring about disorder in the economy, 

 through the absence of a sufficiently rapid discharge of the matters ex- 

 creted into the alimentary canal. Hence cellulose and like substances, 

 even when unutilized through absorption, are not without their use, and 

 experience shows that digestion may be promoted by eating undigestible 

 things. 



466. The several food-stuffs of a diet may be drawn from the animal 

 or from the vegetable kingdom. Vegetable proteids appear to undergo the 

 same changes in the alimentary canal as do animal proteids, and the main 

 effects on the body of proteids from the two sources seem to be the same. 

 Our knowledge at present, however, is too imperfect to enable us to decide 

 whether the functions of the two are exactly the same, whether the body 

 behaves exactly the same upon a diet in which the proteids are exclusively 

 of vegetable origin, as upon a diet in which, otherwise the same, the pro- 

 teids are partly of animal origin also. Nor have we much better know- 

 ledge of the relative nutritive value of vegetable and animal fats. And as 

 we have already said, we possess little or no exact knowledge as to the part 

 played by those extractives in respect to the amount and nature of which 

 animal food strikingly differs from vegetable food. In attempting, there- 

 fore, a judgment from a purely physiological point of view as to the value 

 of an exclusively vegetarian diet compared with a diet of both animal and 

 vegetable origin, we can do little more at present than inquire whether the 

 former supplies the several food-stuffs in adequate quantity, in proper pro- 

 portion, and in such a form as to be economically utilized by the body. 



The careful examination during three separate periods of several days 

 each of the ingesta and egesta of a man, 28 years old, weighing 57 kilos, 

 who had for three years lived on an exclusively vegetable diet, viz., bread, 

 fruit, and oil, gave the following results : 



The daily diet consisted on the average of 719 grms. solid matter and 

 1084 grms. water. It contained 



Proteids 54 grammes, containing 8.4 N. 



Fats 22 grammes. 



Carbohydrates 557 grammes (about J sugar and \ starch). 



(Cellulose) 16 grammes. 



