ssed in calories. A ration for a laborer at active out-of-door work
should yield about 3200 calories. The calory is the unit of heat, and
represents the heat required to raise the temperature of a kilogram of
water 1 deg. C., or four pounds of water 1 deg. F. The caloric value of
foods is determined by the calorimeter, an apparatus which measures heat
with great accuracy. A pound of starch, or allied carbohydrates, yields
1860 calories, and a pound of fat 4225 (see Section 13). While a gram of
protein completely burned produces 7.8 calories, digested it yields only
about 4.2 calories, because, as explained in the preceding section, not
all of the carbon and oxygen are oxidized.[59] The caloric value or
available energy of a ration can be calculated from the digestible
nutrients by multiplying the pounds of digestible protein and
carbohydrates by 1860, the digestible fat by 4225, and adding the
results. For determination of the available energy of foods under
different experimental conditions, and where great accuracy is desired,
a specially constructed respiration calorimeter has been devised, which
is built upon the same principle as an ordinary calorimeter, except it
is large enough to admit a person, and is provided with appliances for
measuring and analyzing the intake and outlet of air.[74] The heat
produced by the combustion of the food in the body warms the water
surrounding the calorimeter chamber, and this increase in temperature is
determined by thermometers reading to 0.005 of a degree or less.

[Illustration: FIG. 55.--CALORIMETER.]

228. Normal Digestion and Health.--While the process of digestion has
been extensively studied, it is not perfectly understood. Between the
initial compounds of foods and their final oxidation products a large
number of intermediate substances are formed, and when digestion fails
to take place in a normal way, toxic or poisonous compounds are produced
and various diseases result. It is probable that more diseases are due
to imperfect or malnutrition than to any other cause. There is a very
close relationship between health and normal digestion of the food.

The cells in the different parts of the digestive tract secrete fluids
containing substances known as soluble ferments, or enzymes, which act
upon the various compounds of foods, changing them chemically and
physically so that they can be absorbed and utilized by the body. (See
Section 31.) Some of the more important ferment