of foods that contains nitrogen in any considerable quantity.
There can be no absolute necessity for any other food-stuffs but those
containing the proteid and mineral elements of the body. From what has
been said, it will readily be seen that whether an animal be carnivorous
or herbivorous, it begins to starve as soon as its vital food-stuffs
consist only of amyloids, or fats, or both. It suffers from what has
been termed _nitrogen starvation,_ and if proteid matters are withheld
entirely, it soon dies. In such a case, and still more in the case of an
animal which is entirely deprived of vital food, the organism, as long
as it continues to live, feeds upon itself, the waste products
necessarily being formed at the expense of its own body.

Although proteid matter is the essential element of food, and under
certain circumstances may be sufficient of itself to support the body,
it is a very uneconomical food. The white of an egg, which may be taken
as a type of the proteids, contains about fifteen per cent. of nitrogen,
and fifty-three per cent. of carbon; therefore, a man feeding upon this,
would take in about three and a half times as much carbon as nitrogen.
It has been proved that a healthy, adult man, taking a fair amount of
exercise and maintaining his weight and body temperature, eliminates
about thirteen times as much carbon as nitrogen. However, if he is to
get his necessary quantity, about 4000 grains of carbon, out of albumen,
he must eat 7,547 grains of that substance; but this quantity of albumen
contains nearly four times as much nitrogen as he requires. In other
words, it takes about four pounds of lean meat, free from fat, to
furnish 4,000 grains of carbon, the quantity required, whereas one pound
yields the requisite quantity of nitrogen. Thus a man restricted
exclusively to a proteid diet, must take an enormous quantity of it.
This would involve a large amount of unnecessary physiological labor, to
comminute, dissolve, and absorb the food, and to excrete the superfluous
nitrogenous matter. Unproductive labor should be avoided as much in
physiological as in political economy. The universal practice of
subsisting on a mixed diet, in which proteids are mixed with fats or
amyloids, is therefore justifiable.

Fats contain about 80 per cent. of carbon, and amyloids about 40 per
cent. We have seen that there is sufficient nitrogen in a pound of meat
free from fat, to supply a healthy adult man for twenty-four