ON DIET. 517 



is not only complex but variable ; so complex and variable that simple ex- 

 perience is at present a more trustworthy guide than speculative physiology. 

 We may add that the physiological action of alcoholic drinks is still further 

 complicated by the fact that most such drinks contain, beside ethylic alcohol, 

 various other allied substances, whose action is even more potent than that 

 of the ethylic alcohol itself, and whose presence very markedly determines 

 the total effect of the drink. Such articles of diet as tea and coffee stand 

 upon very much the same footing as alcohol. 



The quantity of fluid which a man drinks or should drink daily, or 

 more correctly the quantity of water which he should daily add to the 

 dry solids of his diet, must vary widely according to circumstances. It 

 will differ according as he is perspiring freely or not, according to the 

 nature of the dry solids of the diet, whether largely carbohydrate or not, 

 and so on. A lower limit, below which excretion is 'impeded, and a higher 

 limit, above which digestion and metabolism are injuriously affected, prob- 

 ably exist ; but we have as yet no adequate data which will enable us to fix 

 either of them. 



465. In the selection of articles of food to supply the food-stuffs and 

 other constituents of a normal diet, regard must, of course, be had in the 

 first place to the amount of potential energy present in the material. The 

 articles chosen for the daily fare must contain between them so much pro- 

 teid, fat, and carbohydrate, representing so much available energy. But 

 it is no less important that the potential energy in the material should be 

 really available for the economy. The material must have such qualities 

 that it is digested within the alimentary canal, and further that its digestion 

 and absorption do not give rise to trouble either in the alimentary canal or 

 in that secondary digestion carried on by means of the various metabolic 

 events which we have discussed in preceding sections. A really nutritious 

 substance is one which not only contains in itself an adequate supply of 

 energy, but is of such a nature that its energy can be appropriated by the 

 economy with ease, or at least with as little trouble as possible. We have 

 approximate data for determining how far an estimate of the relative use- 

 fulness of various articles of food must be corrected, by allowing for the pro- 

 portion of each which after an ordinary meal merely passes through the 

 alimentary canal, and the energy of which is not in any way available for 

 the body's use. Thus, a number of observations carried out on healthy 

 individuals gave, in the case of the following articles of food, the following 

 figures as the percentage, reckoned in each case on dry material, which 

 could be recovered from the feces, and was, therefore, not digested and not 

 used by the body : Meat, 5 per cent. ; eggs, 5 per cent. ; milk, 9 per cent. ; 

 bread (white), 4 per cent. ; black bread, 15 per cent. ; rice, 4 per cent. ; 

 maccaroni, 4 per cent. ; maize, 7 per cent. ; peas, 9 per cent. ; potatoes, 11 

 per cent. It must, however, be remembered that the actual correction to be 

 made in any case will depend on the mode of cooking of the material, on 

 the character of the meal of which it forms part, and on the individual 

 capabilities of the consumer, the latter, too, varying under different circum- 

 stances. 



The above refers to what may be called rough digestibility, but besides 

 this there are other circumstances to be considered. The same food-stuff 

 in two articles of food, though actually digested, that is to say taken up by 

 the alimentary canal, may, even while still within the alimentary canal, 

 undergo changes in the one case differing from those in the other. A pro- 

 teid may for instance in one case tend to be entirely converted into peptone, 

 or to break up into leucin, etc., or in other cases to undergo other changes ; 

 and a carbohydrate may in one case be absorbed as maltose, and in another 



