Finally, we cannot avoid the conclusion that the outgoings of the body, in the form of matter and energy, are subject to great variation, incidental to the degree of activity of the day or hour. The ordinary vicissitudes of life, bringing days of physical inaction, followed perhaps by periods of unusual activity; changes in climatic conditions, with their influence upon heat production in the body; alterations in the character and amount of the daily dietary, etc.,--all seemingly combine as natural obstacles to the maintenance of a true nutritive balance. Outgo, however, must be met by adequate amounts of proper intake if there is to be an approach toward a balance of nutrition. In some way the normal, healthy man does maintain, approximately at least, a condition of balance; not necessarily for every hour or for every day, but the intake and outgo if measured for a definite period, not too short, say for a week or two, will be found to approach each other very closely. Body equilibrium and approximate nitrogen balance may be reasonably looked for, as well as a balance of total energy, in the case of a healthy man leading a life which conforms to ordinary physiological requirements. The man who, on the other hand, consciously or unconsciously, continues an intake way beyond the outgo, whose daily income of nitrogen and total fuel value far exceeds the requirements of his body, obviously lives with an accumulating plus balance, which ordinarily shows itself in increasing body-weight and with a storing away of fat. Equally conspicuous is the effect of an inadequate income of proper nutriment; a food supply which persistently fails to furnish the available nitrogen and total energy value called for by the body under the conditions prevailing, will inevitably result in a minus balance, which, if continued too long, must of necessity tax the body’s store to the danger limit. At the same time, the well-nourished individual, without being unduly burdened by a bulky store of energy-containing material, is always supplied with a sufficient surplus to meet all rational demands, when from any cause the intake fails, for brief periods of time, to be commensurate with the needs of the body. It is reasonable to believe, however, that in the maintenance of good health, and the preservation of a high degree of efficiency, the body should be kept in a condition approaching a true nutritive balance. CHAPTER IV SOURCE OF THE ENERGY OF MUSCLE WORK, WITH SOME THEORIES OF PROTEID METABOLISM