n invented for measuring the heat production of a man, an apparatus
which is called a calorimeter or a measurer of calories. If one puts a
man weighing, say, 156 pounds in the box of such an apparatus, so that
he lies comfortably on a bed in complete muscular relaxation, and before
his breakfast, one finds that he produces 70 calories an hour. Only in
certain types of disease is there any variation from this normal, though
of course the weight of the man makes a difference in his requirement
for energy. If, at the same time the subject is in the box, the quantity
of oxygen which he absorbs is measured and if certain other chemical
analyses be carried out, one can calculate the exact amounts of protein,
fat, and sugar which have been oxidized by this oxygen. Now, if one
calculates how much heat ought to have been set free from the oxidation
of these quantities of protein fat and carbohydrate, it is discovered
that the heat which ought to have been produced is exactly that quantity
which was measured as having been produced by the man. This measurement
represents the _basal metabolism_ of a man at complete rest, when his
oxidative activities are at their lowest ebb.

The basal metabolism as measured by 70 calories per hour in the case of
this individual represents the sum of the fuel needed--(1) to maintain
the beating of the heart, which every minute of a man's life moves the
blood or one-twentieth part of the weight of the body, in a circle
through the blood-vessels; (2) to maintain the muscles of respiration
that the blood may be purified in the lungs; (3) to maintain the body
temperature at that constant level which is so characteristic that a
slight variation signifies illness, and (4) to maintain in the living
state the numerous tissues of the body. Any extraneous muscular
movements are carried out in virtue of an increased oxidation of
materials and the heat production rises above the level of the basal
metabolism with increased muscular effort. For a long time the power for
the maintenance of the human machine can be furnished by its own body
fat, as is seen in cases of prolonged fasting, but usually the power is
derived instead from the food-fuel which is taken. The great question in
the world to-day is whether or not a sufficient quantity of food-fuel is
available to support the human family. The question of calories is not
an academic one, but an intensely practical one.

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