CHAP, v.] NUTRITION. 663 



it is no less important to secure that the energy potential 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 sec- 

 tions. 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 usefulness of various articles of food must be cor- 

 rected by allowing for the proportion 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 fseces, and 

 was therefore not digested and not used by the body : Meat 

 5 p.c., Eggs 5 p.c., Milk 9 p.c., Bread (white) 4 p.c., Black 

 Bread 15 p.c., Rice 4 p.c., Maccaroni 4 p.c., Maize 7 p.c., 

 Peas 9 p.c., Potatoes 11 p.c. 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 capabili- 

 ties of the consumer, the latter too varying under different 

 circumstances. 



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 proteid 

 may for instance in one case .tend to be entirely converted into 

 peptone, or to break up into leucin, &c., or in other cases to 

 undergo other changes; and a carbohydrate may in one case 

 be absorbed as sugar, and in another 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 advan- 

 tageous 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 



