The minimum demand for energy is the amount which is required for the "basal metabolism." The chief factors known to increase the vital activity or metabolism of the body are muscular activity and ingestion of food. If the former is precluded by enforced repose and the latter is eliminated by observing the metabolism when the active processes of digestion have ceased, i. e., with the subject in the post-absorptive state, we obtain a value which may be considered to be the basal metabolism of that individual.

While in this measurement of the basal metabolism the influence of muscular activity and food are eliminated, there are other factors which produce variation in the basal metabolism of normal individuals. Among these the most obvious is body-size, as indicated by height and weight. Age also influences the metabolism, for elderly people of exactly the same height and weight as younger people have a somewhat lower basal metabolism. There is strong evidence that during deep sleep the metabolism is perceptibly lowered. Severe muscular work unquestionably has a stimulating after-effect upon the metabolism which may persist for many hours after the work has ceased. Furthermore, careful measurements of the basal metabolism over a considerable period of time show that variations of no mean magnitude occur even with the same individual, these averaging at times as high as 13 to 14 per cent.1 The exact cause for these variations is not known. That they are seasonal is highly improbable. Suitable analysis of the correlations for weight, height, surface, age, and other factors which influence the metabolism can only be made from measurements obtained with a large number of normal individuals.2