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886 METABOLISM.
The proteins act differently from the fats and carbohydrates. They
are only incompletely burnt, and they yield certain decomposition prod-
ucts, which, leaving the body with the excreta, still represent a certain
quantity of energy which is lost to the body. The heat of combustion
of the proteins is smaller within the organism than outside of it, and they
must therefore be specially determined. For this purpose Rubner 1
fed a dog on washed meat, and he subtracted from the heat of combustion
of the food the heat of combustion of the urine and feces, which cor-
responded to the food taken plus the quantity of heat necessary for the
swelling up of the proteins and the solution of the urea. Rubner has
also tried to determine the heat of combustion of the proteins (muscle-
proteins) decomposed in the body of rabbits in starvation. According
to these investigations, the physiological heat of combustion in calories
for each gram of substance is as follows:
1 gram of the dry substance Calories.
Protein from meat 4.4
Muscle 4.0
Protein in starvation 3.8
Fat (average for various fats) 9.3
Carbohydrates (calculated average) 4.1
The physiological combustion value of the various foods belonging to
the same group is not quite the same. It is, for instance, 3.97 calories
for a vegetable protein, conglutin, and 4.42 calories for an animal protein
body, syntonin. According to Rubner the normal heat value per 1
gram of animal protein may be considered as 4.23 calories, and of vegetable
protein as 3.96 calories. When a person on a mixed diet takes about
60 per cent of the proteins from animal foods and about 40 per cent from
vegetable foods, the value of 1 gram of the protein of the food is equivalent
to about 4.1 calories. The physiological value of each of the three
chief groups of organic foods, by their decomposition in the body, is in
round numbers as follows:
Calories.
1 gram protein 4.1
1 gram fat 9.3
1 gram carbohydrate 4.1
1 gram alcohol 7.1
These figures are generally used in the calculation of the energy con-
tent of various foodstuffs and diets.
The extent of gas exchange and the so-called respiratory quotient
is, besides the extent of nitrogen elimination, of the greatest importance
in the calculation of the extent of energy metabolism and the division
of the energy between the protein, fat and carbohydrate.
< >n comparing the inspired and expired air we learn, on measuring
them when dry and at the same temperature and pressure, that the volume
1
Zeitschr. f . Biologie, 21.
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