DIETETICS. 817 



It must be borne in mind, however, that the entire nutritional value 

 of a food is not expressed in its heat value some of our food mate- 

 rial the green foods and fruits, for example are useful and in a 

 measure essential because of their salts and organic acids. In a 

 general way, however, the heat energy of a food expresses its value 

 as a means for maintaining the body in a normal condition. 



Dietetics. The subject of the proper nourishment of individ- 

 uals or collection of individuals in health and in sickness is treated 

 usually in works upon hygiene or dietetics. The practical details 

 of the preparation and composition of diets must be obtained from 

 such sources.* The general principles upon which practical diet- 

 ing depends have been obtained, however, from experimental work 

 upon the nutrition of man and the lower animals, some account of 

 which has been given in the foregoing pages. In a healthy adult the 

 main objects of a diet are to furnish sufficient nitrogenous and non- 

 nitrogenous foodstuffs, salts, and water to maintain the body in equi- 

 librium of material and of energy that is, the diet must furnish the 

 material for the regeneration of tissue, and the material for the heat 

 produced and the muscular work done. Nutritional experiments 

 prove that this object may be accomplished by proteid food alone 

 together with salts and water. It is doubtful, however, whether, 

 in the case of man, such a diet could be continued for long periods 

 without causing some nutritional disturbance, directly or indi- 

 rectly. It will be remembered that a pure meat diet is not entirely 

 proteid, since all flesh contains some fats and carbohydrates (gly co- 

 gen). The functions of a diet are accomplished more easily and 

 more economically when it is composed of proteids and fats, or pro- 

 teids and carbohydrates, or, as is almost universally the case, of 

 proteids, fats, and carbohydrates. The experience of mankind 

 shows that such a mixed diet is most beneficial to the body and 

 most satisfying to that valuable regulating mechanism of nutrition, 

 the appetite. The proportions in which the proteids, fats, and 

 carbohydrates are mixed in a diet vary greatly among different 

 nations and individuals. So far as the fats and carbohydrates are 

 concerned, their use is mainly that of fuel to supply energy, and 

 from this standpoint we ought to be able to exchange them in the 

 diet in the ratio of their heat values. 



This ratio, or as it is frequently called, the isodynamic equiva- 

 lent, is as 9.3 to 4.1 or 2.3 to 1, and within the limits permitted by 

 the appetite we should be able to substitute 1 part of fat for 2.3 

 parts of sugar or starch. Experiments upon animals as well as 

 the experience of mankind show that this substitution can be made. 



* For practical directions see Gautier, " L'alimentation et les regimes," 

 1904; Blyth, "Foods : their Composition and Analysis." 

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