DIETETICS. 943 



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 equiUbrium 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 and other work done. The functions of a diet are accom- 

 plished most easily and most economically when it consists of pro- 

 teins and fats, or proteins and carbohydrates, or, as is almost uni- 

 versally the case, of proteins, fats, and carbohydrates, together 

 with an adequate quantity of water, the necessary inorganic salts, 

 and the newly discovered food accessories (vitamins). The expe- 

 rience of mankind shows that such a mixed diet is most beneficial 

 to the body and most satisfying to that valuable regulating mech- 

 anism of nutrition, the appetite. Expressed in its most general 

 form the cells of our body need food for two purposes: First, to 

 supply the energy needs; second, to furnish the material for the 

 construction of their own living substance, that is, for assimilation. 

 The first of these purposes is fulfilled by any of the three energy- 

 yielding foodstuffs, carbohydrates, fats, or proteins, but as a matter 

 of fact we use chiefly the carbohydrates on account of their economy 

 and the ease with which they are utilized by the body. For the 

 second purpose, the construction of protoplasm or living matter 

 proteins (or their cleavage products, the various amino-bodies) are 

 absolutely necessary, and also probably an adequate supply of 

 inorganic salts and the so-called accessories (vitamins) . In accord- 

 ance with this specific and necessary function of the protein we find 

 that the amount used in the daily diet is fairly constant, about 100 

 gms., while the proportions of fat and carbohydrate show wide 

 variations. Since from the energy standpoint the fats and carbo- 

 hydrates have a common function, serving as fuel for the energy 

 needs of the body, 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 to a certain extent, although it is not advisable to eliminate 

 either of these foodstuffs entirely from the diet. The fact that 

 within certain hmits fats and carbohydrates may be substituted 

 for each other is illustrated in a general way by the different diets 

 recommended by various physiologists, since it will be noticed 

 that in those in which the proportion of fat is large the amount of 

 carbohydrate is reduced. 



