In determining the nutritive value of aliments by the study of their
chemical composition, we cannot adhere strictly to the results furnished
by analysis. For, says Baron Liebig, we cannot reckon upon results in the
human stomach with the same regularity as we would in the alembics of our
laboratories.
Physiologists divide alimentary substances into two classes: the
nitrogenous, which, according to Dumas, supply the demands of
assimilation, and the non-nitrogenous, which are called by Liebig
respiratories, from furnishing the products consumed by respiration.
Neither the one nor the other will alone support life indefinitely, and
when one or the other decreases below well-defined limits, health
declines, and finally life becomes extinct from inanition.
Milne Edwards gives, as the mean amount of these two classes, required for
all climates, not less than three hundred and fifteen grains of nitrogen
and thirty-three hundred and fifty grains of carbon in the twenty-four
hours. These views are adopted by most physiologists; yet the analyses of
Schlossberger and Kemp indicate that the idea of estimating the value of
food by the quantity of nitrogen it contains is a fallacious one.
The beautiful experiments of Bernard and the modern physiologists have
unfolded many of the laws that regulate digestion and assimilation. Yet
the human researches in the great arcana of nature are extremely limited,
in comparison with the vast range of physical phenomena, and every day we
are reminded of the remarks of Boerhaave to his students: "Let all these
heroes of science meet together; let them take bread and wine, the food
which forms the blood of man, and by assimilation contributes to the
growth of the body; let them try by all their art, and assuredly they will
not be able from these materials to produce a single drop of blood,--so
much is the most common act of nature beyond the utmost efforts of the
most extended science."
The composition of the typical food of nature is revealed to us in the
analysis of human milk.