848 NUTRITION AND HEAT REGULATION. 



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 an 

 equilibrium 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 experi- 

 ments prove that this object may be accomplished by protein 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 

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

 entirely protein, since all flesh contains some fats and carbohydrates 

 (glycogen). The functions of a diet are accomplished more easily 

 and more economically when it is composed of proteins and fats, or 

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

 proteins, 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 proteins, 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." 



