POTENTIAL ENERGY OF FOOD. 



847 



portion is given off in the feces. Rubner has calculated the avail- 

 able heat energy of proteins by direct experiments upon animals. 

 In these experiments the heat value of the protein fed was directly 

 determined by burning a sample in a calorimeter. Then after feed- 

 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. proteid = 4100 calories (4.1 C.). 



1 gm. carbohydrate = 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, on the 

 belief that, for the most part, our food is used as fuel to supply 

 energy to the body. These values for some of our ordinary foods 

 are as follows : * 



PROTEID. FAT. 



Beefsteak, porterhouse 19.1 17.9 



Beefsteak, round (lean) 20.2 2.4 



Corned beef (canned) 26.3 18.7 



Veal, leg (lean) 19.4 3.7 



Veal liver 19.0 5.3 



Mutton, leg (lean) 16.5 10.3 



Pork, ham (fresh, lean) 24.8 14.2 



Pork chops, medium fat 13.4 24.2 



Chicken (fowl) 13.7 12.3 



Shad 9.4 4.8 



Shad roe 20.9 3.8 



Eggs 11.7 10.7 



Milk 3.3 4.0 



Oatmeal 16.1 7.2 



Rice 8.0 0.3 



Wheat flour (entire wheat) 13.8 1.9 



Green peas 7.0 0.5 



Potatoes (raw) 2.2 0.1 



Spinach . 2.1 0.3 



Tomatoes 0.9 0.4 



Apples 0.4 0.5 



Bananas 1.3 0.6 



CARBOHY- 

 DRATE. 



2.6 



*5.0 



67.5 



79.0 



71.9 



16.9 



18.4 



3.2 



3.9 



14.2 



22.0 



ASH. 



0.8 

 1.2 

 4.0 

 1.1 

 1.3 

 0.9 

 1.3 

 0.8 

 0.7 

 0.7 

 1.5 

 0.7 

 0.7 

 1.9 

 0.4 

 1.0 

 1.0 

 1.0 

 2.1 

 0.5 

 0.3 

 0.8 



HEAT VALUE 

 IN CALORIES 

 PER POUND. 



1110 



475 

 1280 



520 



575 



740 

 1060 

 1270 



775 



380 



600 



680 



325 

 1860 

 1630 

 1675 



465 



385 



110 



105 



290 



460 



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

 United States Department of Agriculture, 1899. 



