Alimentary Equilibrium. Proteins and Calories. Dietary Standards. Army and Navy Dietaries. Prison Dietaries. Diet of Savages. Investigations of Actual Dietaries in the United States.

Equilibrium

In order to determine the quantity of food that is needed to supply the waste that man's tissues constantly undergo and to supply fuel for the generation of muscular force and bodily heat it is necessary to study carefully the income and the outgo of the body. If the absorbed matter equals in weight the excreted matter, the body will neither gain nor lose: it will be in equilibrium or balance. This condition cannot long be maintained accurately. We habitually eat more than is needed to supply waste. Children must eat much more proportionately than adults, for they must provide for growth as well as for wear and tear. This habit of eating very generously is carried into adult life and excites many of the digestive disturbances that arise at that time. It also helps to produce the obesity that is so common in middle age. In adult life only so much food is needed as is required to supply energy and to repair waste. In old age less is needed, for less force is exerted and there is a gradual waste of tissue and consequent loss of flesh.

Nitrogen equilibrium exists when the amount of nitrogen in the food eaten equals the amount of nitrogen in the excreta. This condition can be brought about and even maintained for some time by due care. It is attained, however, only as an experiment. By such studies we have learned that the average man excretes approximately twenty grams of nitrogen daily. The amount of nitrogen destroyed by life's processes is greater when the diet is chiefly nitrogenous than when it is mixed and contains a generous proportion of fat and carbohydrate. Therefore, these last are often spoken of as nitrogen savers. Ordinarily they furnish the fuel needed to generate muscular power and heat, but if they are insufficiently represented in a diet, the nitrogenous tissues and food must supply this fuel.

Carbon equilibrium also can be established, and the experiment teaches that approximately 320 grams of carbon are used daily by the average adult.

Catabolism

The catabolism or protein is nearly independent of muscular work, for it repairs cellular waste almost exclusively. It has been found, however, that nitrogen equilibrium can be maintained by a supply of nitrogenous food varying between comparatively wide limits. This means that nitrogenous food in greater quantities than are strictly needed to maintain nitrogen equilibrium will cause a waste of tissues as well as repair. In other words, all changes are stimulated by proteins. Moreover, when a change is made in the amount of protein eaten it requires a few days to bring about protein balance under the new conditions. Under normal conditions muscular work depends mainly, if not exclusively, on the oxidation of nonnitrogenous material.