940 NUTRITION AND HEAT REGULATION. 



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 abso- 

 lutely necessary.* In accordance with this specific and neces- 

 sary function of the protein we find that the amount used in the 

 daily diet is fairly constant, about 100 gms., while the propor- 

 tions of fat and carbohydrate show wide variations. Since from 

 the energy standpoint the fats and carbohydrates 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, although it is not advisable to eliminate either of these 

 foodstuffs entirely from the diet. The fact that within certain 

 limits fats and carbohydrates may be substituted for each other 

 is illustrated in a general way by the different diets recom- 

 mended by various physiologists, since it will be noticed that in 

 those in which the proportion of fat is large the amount of carbo- 

 hydrate is reduced. 



AVERAGE DIETS AND THEIR HEAT VALUES. 



MOLESCHOTT. RANKE. VoiT. 



CALORIES. CALORIES. CALORIES. 



Protein 130 gms. . . . 533 100 gms. . . . 410 118 gms. . . . 483- 



Fats 40 "... 372 100 ' ... 930 56 "... 520 



Carbohydrates ... 550 ' . . . 2275 240 ' . . . 984 500 ' . . . 2050 



2980 2324 3053 



FORSTER. ATWATER. 



CALORIES. CALORIES. 



Protein 131 gms. ... 567 125 gms 512 



Fats 68 "... 632 125 " .... 1172 



Carbohydrates . . 494 ' . . . 1825 400 " ... . 1640 



2024 3324 



The average heat value of these diets is equal to 2742 calories,. 

 of which about 18 per cent, is furnished by the protein. Generally 

 speaking, it will be found that in the dietaries sele'cted voluntarily 

 by mankind the protein furnishes from 15 to 20 per cent, of the 

 total heat value of the diet. According to some physiologists 

 this proportion is unnecessarily large and it might be reduced 

 to as little as 5 or 10 per cent. Whether or not such a change is. 

 * See p. 897 for a possible amendment to this general statement. 



