POTENTIAL ENERGY OF FOOD. 941 



ing a known amount of the protein the urine and feces were col- 

 lected and their heat value was determined in the same way. The 

 difference between the total heat value of the protein fed and the 

 heat value lost in its excreted products in the feces and urine gave 

 the actual heat energy obtained from the protein by the animal 

 body. Results obtained by this method give an average value 

 for 1 gm. protein of 4100 calories (4.1 C), or, since protein contains 

 an average of 16 per cent, of nitrogen, we may say that 1 gm. of ni- 

 trogen ingested as protein has a heat value of 4.1 X 6.25 = 25.6 C. 

 The figures that are used, therefore, in estimating the heat value 

 of our foodstuffs are: 



1 gm. protein = 4100 calories (4.1 C). 



1 gm. carbohydrate (starch) = 4100 calories (4.1 C). 

 1 gm. fat = 9305 calories (9.3 C). 



Making use of these values, it is obvious that we can calculate the 

 total heat value of any given diet. If we analyze the food for its 

 composition in the three principal foodstuffs we may determine how 

 many calories will be furnished to the body. In many of the tables 

 published to show the composition of the different foods figures are 

 given also to express their heat value or potential energy, since 

 these substances taken into the body as food leave it in the form of 

 water, carbon dioxid, and urea (or one of the related nitrogenous 

 excreta). So that all of the potential chemical energy contained 

 in them that can be liberated by oxidation to these stages must 

 have been set free in the body in one form or another. It is con- 

 venient to express this total liberation of energy in terms of its 

 heat equivalent. Indeed, for the most part, the potential chemical 

 energy of the food does take the form of heat, mediately or im- 

 mediately, as the result of the processes going on in the tissues, 

 and in this form it is given off from the body to the external world. 

 No measurable amount of energy is given off from the body except 

 as heat and mechanical work. When the body is at rest all of the 

 energy is given off as heat, and if this is measured in calories it will 

 be found to equal the heat equivalent of the proteins, fats, and 

 carbohydrates oxidized in the body during the period of obser- 

 vation. The values for some of our ordinary foods are shown in 

 the table on page 942.* 



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 material^the green foods and fruits, for example — are useful 

 and, indeed, essential because of their salts, organic acids, and 



* Selected from Atwater and Bryant, Bulletin 28 (revised edition), United 

 States Department of Agriculture, 1889. 



