For while no two individuals are exactly alike, there are factors which govern or influence the food requirements of all, and thus make it possible to estimate the needs of the body with a fair degree of accuracy.
It has been found, by means of calorimeter experiments (direct and indirect), that a certain amount of heat is produced within the body, regardless of external movement or food; that is, when a body is lying absolutely quiet with no movement save that of breathing, the internal work of the organism, which is continuous, releases so much heat, and this is produced whether there is food to replace it or whether the body structure is burned. This is known as the ~basal rate of metabolism~, and constitutes the normal ~basal requirements~. Any external movement will increase this rate; the greater the activity the higher the increase. Consequently external work calls for food in addition to that which is used to run the engine, in order to save the body from destruction.
DuBois[20] finds "Basal Metabolism above normal in exophthalmic goiter, in fevers, in lymphatic leukemia, and in pernicious anemia, in severe cardiac disease, and in some cases of severe diabetes and cancer; it is lower than normal in cretinism, and in myxedema, in old age, in some wasting diseases and perhaps in some cases of obesity." This fluctuation in the Basal Rate of metabolism furnishes a factor in the diagnosis of disease, not only recognized but coming more and more in use.
FACTORS DETERMINING THE FOOD REQUIREMENTS OF THE BODY
~For the Adult.~--Muscular activity, Age and Size, are most important factors influencing the food requirements. The physical condition and environment of the individual also exert a certain amount of influence upon the intake of food.
~Work.~--Muscular activity, as already stated, increases the body expenditures; consequently the more active the work the greater amount of energy food needed per unit of weight.
~Age.~--As the individual grows older, the rate of metabolism decreases until, in old age, it is not more than a third to a fifth of what it was in earlier life. This is due to a general "slowing down" of the machinery, the heart does not beat so rapidly, nor is the respiration so quick. The digestive organs, the heart, the liver, and the kidneys, cannot handle the volume of food which was required during the period of greatest physical activity. Hence, any great excess over and above that which is needed for the maintenance of the body in health will be a source of danger to the elderly person. Von Noorden claims the food requirements of individuals from