not our food is equal to our wants. Very naturally the energy
requirements of any individual are influenced by his weight and by
the work he does. But we may take as a standard the results of an
extensive study of American families which indicate that women
require four-fifths as much energy-yielding food as men. It also
seems safe to conclude that a woman weighing 130 pounds who does her
own housework requires food every day having an energy-value of 2,500
calories; smaller women and those who do no work require somewhat
less. In a mixed diet the chief source of this energy--and the source
from which it is most economically obtained--is the carbohydrates.

_Fat_ yields more energy and heat than does carbohydrate, bulk
for bulk; but fat is burned by our tissues less readily. We
instinctively avoid eating a great deal of this food-stuff; in the
course of a day the average person consumes no more than one or two
ounces. The natural aversion which many feel toward fat may possibly
depend upon the difficulty with which they assimilate it. In colder
climates, however, we know fat to be a staple article of diet; and it
is not unlikely that the very conditions which make it necessary
there explain the unusual tolerance for it.

Fat is more than fuel. Deposited in our bodies, beneath the skin for
example, it prevents the escape of heat that we generate and protects
us against the penetration of cold. This food-stuff, therefore,
contributes in several ways toward maintaining the temperature of the
body at a constant level.

Our source of fat is chiefly animal food and in a smaller measure
vegetables; but the fat our food contains is not altogether
responsible for the fat in our bodies. Carbohydrates, if in excess of