protein for protein is absolutely necessary to build and repair tissue,
and carbohydrates cannot do that. But fats and carbohydrates are
interchangeable as fuel or energy foods.

_Calories Needed per Day for Normal Individuals_

[Sidenote: _Business of Growing_]

This depends upon age, weight, and physical activities; the baby and the
growing child needing many more calories per pound per day than the
adult, who has to supply only his energy and repair needs. The aged
require still less than the young adult. As to weight; I have told you
why overweight individuals need so little. As to physical activities;
the more active, obviously the more calories needed, for every movement
consumes calories.

[Sidenote: _Many Know Nothing of This_]

The Maine lumbermen, for instance, while working during the winter
months, consume from 5000 to 8000 Calories per day. But they do a
tremendous amount of physical work.

_Mental work does not require added nourishment._ This has been proved,
and if an excess be taken over what is needed at rest (if considerable
exercise is not taken while doing the mental work) the work is not so
well done.

[Sidenote: _Calories Required for Normal_]

Per pound