The fact is almost self-evident that, as food supports bodily activity and growth, the necessary amount of nutrition must vary greatly with different classes of persons. It is, therefore, no less important than interesting to understand the relation of age, size, sex, disposition, occupation, and other conditions to nutritive demands. Fortunately, through the use of the respiration calorimeter, considerable reliable data have been secured concerning the influence of these factors.

210. Childhood

At no period of life is gaseous exchange (food oxidation) so vigorous or so large in proportion to weight as during childhood. Children are peculiarly active, being constantly in motion during their waking hours. It has been found that a child two and one-half years old, weighing 25 pounds uses, when at rest, half as much oxygen as an adult weighing 150 pounds, and nearly three times as much for each unit of weight. This means that the demand for food energy would be in these proportions, which for young children of varying ages is from 2 to 3 times as much per unit of weight as it is for adults.

The following table shows very clearly how age affects metabolic activity:

Table XXX. Oxygen Use Per Minute (Energy Requirement) For Persons Of Different Ages

It appears that, whether we consider weight or body surface, the child appropriates more oxygen, that is, gives off more heat per unit of weight or of surface, than either the adult or the aged. Besides, the rapidly developing child stores in his tissues protein and inorganic salts which must come from the food. For these reasons the liberal, and sometimes seemingly excessive, amounts of food eaten by children, especially between the ages of ten and sixteen, are not irrational, and those who dictate school dietaries should keep these facts in mind.