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ISODYNAMICS OF FOOD-STUFFS. 891
posed body and foods. Atwater and his collaborators x
have made some
very thorough investigations on this subject on men. In their experi-
ments they made use of a large respiration calorimeter, which not only
exactly determined the excreta, but also made a calorimetric determina-
tion of the heat given out by the person experimented upon, i.e., the work
performed. From the results of these experiments they found an almost
absolutely complete agreement between the calories found directly and
those calculated.
This isodynamic law is of fundamental value in the study of metabo-
lism and nutrition. The quantity of energy in the transformed foods
or the constituents of the body may be used as a measure for the total
consumption of energy, and the knowledge of the quantity of energy
in the foods must also be the basis for the calculation of dietaries for
human beings under various conditions.
The isodynamic theory has been accepted by a large number of inves-
tigators, but not by all. Certain of them, especially the French, accept
an isoglucosic instead of the isodynamic. According to this theory the
organism for its physiological functions can use glucose only, and as a
formation of glucose is possible from proteins as well as fats, those quan-
tities of food-stuffs are to be considered as equivalent which yield an
equal amount of glucose.
The heat value of a foodstuff can be directly determined in a calorim-
eter, but may also be calculated from its composition. If one subtracts
from the gross heat value of the food obtained in one way or another
the combustion heat of the feces and urine with the same diet, there is
obtained the net calorific value of the diet. This value, calculated in
percentage of the total energy content of the food, is called the physio-
logical availability by Rubner.2
In order to elucidate this we will give
a few of Rubner’s values. The loss in calories, as well as the physio-
logical availability, is calculated in percentages of the total energy
content of the food.
p00(j r
Loss in per cent. Total loss Availability
In Urine. In the Feces, in per cent, in per cent.
Cow’s milk 5.13 5.07 10.20 89.8
Mixed diet (rich in fat) 3.87 5.73 9.60 90.4
Mixed diet (poor in fat) 4.70 6.00 10.70 89.3
Potatoes 2.00 5.60 7.60 92.4
Graham bread 2.40 15.50 17.90 82.1
Rve bread 2.20 24.30 26.50 73.5
Meat 16.30 6.90 23.20 76.8
In order to simplify the calculation of the energy exchange there exist other
standard factors, besides the above-mentioned standard figures for the physiological
1
Bull, of Dept. of Agric, Washington, 44, 63, 69, and 109, and Ergebnisse der
Physiologie, 3.
2
Zeitschr. f. Biologie, 42.
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