INTRODUCTION 



The present-day tendency in matters of diet is to stress the vita- 

 mines and minerals, and yet our main need for food is to secure 

 energy, since energy or heat is just as necessary to run the human 

 machine as it is to run any prime motor. The caloric value to the 

 body of a food depends (1) upon the potential energy in the food as 

 eaten, i. e., upon its heat of combustion; (2) upon the energy leaving 

 the body in the form of undigested matter in feces and urine; and 

 (3) upon the energy used by the body in digesting and assimilating 

 the food, i. e., the "cost of digestion." Although the second and the 

 third of these factors will vary somewhat according to the character 

 of the food eaten, the second is relatively so constant with humans 

 (about 8 per cent of the total potential energy) and the third is so 

 small (on the average about 6 per cent) that they may be neglected 

 in this discussion and we may concentrate our attention upon the 

 first factor, the heat of combustion, considering this as representative 

 of the food's caloric value. 



To determine the heat of combustion has required, in the past, an 

 expensive apparatus, manipulated with difficulty, the so-called "bomb 

 calorimeter." The technique of this complicated machine, which gives 

 extraordinarily accurate results, has been mastered by relatively few 

 chemists and physicists. Calculation of the results also is usually 

 a complicated procedure. Chemical analyses of the amounts of pro- 

 tein, fat, and carbohydrate in a food and calculation of the total heat 

 of combustion from the known average heats of combustion of these 

 three main nutrients are another means of determining the energy in 

 food. But chemical analyses are even more time-consuming and 

 costly than are direct determinations with the calorimetric bomb. 



From comparisons of the heat of combustion of various foods, as 

 directly determined by the bomb calorimeter, and the measured car- 

 hon-dioxide production during such combustion it is now known that 

 the caloric value of a liter of carbon dioxide, although constant for 

 any one group of nutrients, may vary with the different groups as 

 much as 30 per cent (the extremes being 5.04 calories per liter with 

 cane sugar and 6.64 calories with animal fat).l The relationship be- 

 tween the oxygen absorbed during a combustion and the heat liberated 

 is, however, much closer, there being a maximum difference of but 6 

 per cent between the caloric value of a liter of oxygen absorbed in 

 the combustion of cane sugar (5.04 calories) and that during the 

 combustion of animal fat (4.72 calories). Since our food is a mix- 



(1) The caloric value of protein Is not considered in this comparison, 

 since the protein metabolism plays but a relatively small role in the entire 



human metabolism. 



