211. Old Age

With advancing years, generally after the age of 70 or 75 is passed, there is a marked decrease in vitality and bodily activity. The demand for food is correspondingly diminished. Von Noorden2 states, on the basis of exact observations, that the gaseous exchange is found to be About 20 per cent less with old persons than with those in middle life. This refers evidently to the resting condition. There is no definite point at which "old age" begins, and the stage of life at which metabolic activity starts on the down grade varies with different persons.

1 "Metabolism and Practical Medicine," Von Noorden, Vol. I, p. 268. 2 Loc. cit., p. 267.

212. Weight

The amount of food required to sustain persons in a resting condition increases with their weight, though not proportionately, that is, a heavy man doing no work uses more total food than a light one, but he uses less per pound of weight. The energy demand for 24 hours has varied in experimental observations from 9 calories per pound of body weight with large individuals, to 14 calories with small; but it only requires 30 per cent or 40 per cent increase of energy expenditure when the weight doubles. The energy use is more nearly proportional to body surface.

Table XXXI. Effect Of Weight On Energy Use

The above table1 illustrates clearly the facts that have been stated. One exception should be made to these general statements, viz., that increased weight, due to the laying on of fat, does not cause an increase of total energy used by the resting individual. If the obese person is active or does external work, then he uses more energy because more is required to move the body around.