476 DIETETICS. [BOOK n. 



presence of uric acid. A large meal of proteid material must tax 

 the system to the utmost in getting rid of or stowing away the 

 nitrogenous crystalline bodies arising through changes either in 

 the alimentary canal or in the liver. 



One value of fats and carbohydrates lies in their being sources 

 of energy, more than three-fourths of the normal income of poten- 

 tial energy coming from them (p. 458); and, as we have seen, they 

 are ultimate sources of muscular energy as well as of heat. But 

 their great characteristic is that they do not, like proteid food, excite 

 the metabolic activity of the body. Hence, to a far greater extent 

 than is the case with proteid food, they can be retained and stored 

 up in the body with comparative ease. The digested elements of 

 fatty or carbohydrate food which go to form the protoplasm of 

 adipose tissue, become part and parcel of a substance which can 

 perform its metabolism without any explosive expenditure of 

 energy, and which therefore, instead of giving rise to bodies de- 

 manding immediate excretion from the system, can deposit its 

 metabolic products as apparently little, but as we have seen in 

 reality greatly, changed fat. In this way the non-nitrogenous 

 food of to-day is rendered .available for future and even far distant 

 wants. 



In comparing fats with carbohydrates, we can only point to the 

 much greater potential energy of the former than of the latter, 

 weight for weight (see p. 458). 



A diet may be chosen either for the simple maintenance of 

 health, or for the sake of muscular energy, or for fattening purposes. 

 For the first purpose there is, we may suppose, a normal diet ; and 

 in the case of man, instinct and experience have probably not erred 

 far in choosing some such proportions as those given on p. 446. If, 

 as we have urged, all food becomes tissue before it leaves the body 

 as waste product, the dominant principle of all nutrition, and the 

 ultimate tribunal of all questions of diet, must be the individual 

 character of the tissue, the idiosyncrasy of the body. The same 

 mysterious qualities which cause the same blood-plasma to become 

 here a muscle, and there a secreting cell, convert the same food into 

 the body of a man or of a sheep. All the simpler and more general 

 laws of metabolism are made subservient to more intricate and 

 special laws of protoplasmic construction. We can only speak of a 

 normal diet in the same way that we speak of the average intelli- 

 gence of man. 



In seeking to supply such a normal diet out of ordinary 

 articles of food, we must bear in mind that the nutritive value 

 of any substance, estimated in terms of the potential energy of 

 the proteids, fats or carbohydrates it contains, must of course 

 be corrected by its digestibility. One gramme of cheese has, as 

 far as potential energy is concerned, an exceedingly high value ; 

 but the indigestibility of cheese brings its nutritive value to a 



