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CHAPTER XVII.
METABOLISM WITH VARIOUS FOODS, AND THEIR NECESSITY
TO MAN.
I. GENERAL DISCUSSION AND METHODS USED IN THE STUDY OF MATTER
AND FORCE METABOLISM.
The conversion of chemical energy into heat and mechanical work
which characterizes animal life, leads to the formation of relatively
simple compounds—carbon dioxide, urea, etc.—which leave the organism,
and which, moreover, being very poor in energy, are for this reason of
little or no value to the body. It is therefore absolutely necessary for
the continuance of life and the normal course of the functions of the body
that the organism and its different tissues should be supplied with new
material to replace that which has been exhausted. This is accom-
plished by means of food. Those bodies are designated as food which
have no injurious action upon the organism and which serve as a source
of energy and can replace those constituents of the body that have been
consumed in metabolism or that can prevent or diminish the consumption
of such constituents.
Among the numerous dissimilar substances which man and animals
take with the food all cannot be equally necessary or have the same value.
Some perhaps are unnecessary, while others may be indispensable. We
have learned by direct observation and a wide experience that besides the
oxygen, which is necessary for oxidation, the essential foods for animals
in general, and for man especially, are water, mineral bodies, proteins,
carbohydrates, and fats.
It is also apparent that the various groups of food-stuffs necessary for
the tissues and organs must be of varying importance; thus, for instance,
water and the mineral bodies have another value than the organic foods,
and these again must differ in importance among themselves. The
knowledge of the action of various nutritive bodies on the exchange of
material from a qualitative as well as a quantitative point of view must
be of fundamental importance in determining the value of different
nutritive substances relative to the demands of the body for food under
various conditions, and also in deciding many other questions—for instance,
the proper nutrition for an individual in health and in disease.
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