The body-weight is dependent upon the food intake on the one hand and the oxidation of food or body-material on the other. The majority of adults retain their body-weight almost without change for years, if not decades. This is the more surprising when one considers the great number of foods available and the variety in nature, quality, and quantity of foods, these changing with the season and geographical location. Furthermore, even with individuals who live a regular life, the muscular activity varies considerably. With so great changes in the factors influencing the body-weight, this practically constant balance between food intake and consumption of material in the body, with no great storage of material or drafts upon previously stored material, leads one to conclude that the majority of individuals so select their diet as to meet their physiological needs, as far as the total fuel content of the food is concerned.

Since in this monograph the factor of growth is not to be considered, it is only necessary to discuss the food requirement for keeping the body in condition for the needs of daily life. The body-cells are continually undergoing metamorphosis and disintegration and hence must be repaired. In addition there are temporary drafts upon stored body-material. This is obvious when one considers that the combustion of material is continuous, while the food intake is only intermittent. Many individuals take food only twice a day; in the interim, particularly in the early morning hours, the combustion of material in the body must be largely that which has been previously deposited in the form of either glycogen or fat. With exercise these drafts upon body-material may be even greater, but the compensation is rapid; in fact, after severe exercise large amounts of food are usually consumed, so that the delicate balance between intake and output is even then maintained.