THE FOOD AS A SOURCE OF ENERGY. 281 
these estimates is checked by a comparison of the computed with 
the actual heats of combustion of 276 different samples of food, the 
average results showing a close agreement. Assuming the potential 
energy of the urine to be all derived from the proteids, the average 
of 7.9 Cals. per gram nitrogen given above (p. 278) corresponds to 
1.25 Cals. per gram of protein (N X6.25) metabolized. The loss of 
energy in the feces is estimated from a number of digestion experi- 
ments upon single foods, the results being checked by a comparison 
of the actual and computed apparent digestibility in 93 digestion 
experiments on mixed diet. Finally, the proportions of the several 
nutrients which are derived from different classes of foods in 
average mixed diets are computed from the results of 185 dietary 
studies. The final results thus obtained for the metabolizable 
energy or “fuel value” of the nutrients are shown in the table on 
page 282. 
The average results for the ordinary mixed diet of man were— 
Protein........... 22. -eeeeeeee eee 4.0 Cals. per gram 
‘Carbohydrates.................. 40 “& “ « 
Fat... Capes eesenOed sh AE ORE 
These factors are smaller than those proposed by Rubner, largely 
because they relate to the total and not to the digested nutrients. 
Comparisons of the computed with actual metabolizable energy of 
mixed dietaries, using the factors of the above table, gave concor- 
dant results. 
§ 3. Experiments on Herbivora. 
Tue Mécxern Investications.—The larger share of our present 
knowledge regarding the metabolizable energy of the food of her- 
bivora is due to the investigations upon mature cattle which have 
been made by Kellner * since 1894 at the Méckern Experiment 
Station. In the earlier series of experiments (including those by 
G. Kihn, reported by Kellner ¢) additions of commercial wheat 
gluten and of starch were made to a basal ration consisting exclu- 
sively of coarse fodder (hay or straw). In the later series of ex- 
periments additions of the same substances and of oil and beet 
molasses on the one hand, and of coarse fodders on the other hand, 
were made to a mixed basal ration. 
* Landw. Vers. Stat., 47, 275; 50, 245; 58, 1. ft Ibid., 44, 257. 
