A. Scientific Methods Of Inquiry

The discussion of human nutrition on a scientific basis requires an understanding of what are the physiological needs of the human organism under various conditions and how these may be met most efficiently and economically. Before stating present views of the functions of the several nutrients, it will be well to gain some conception of how we have arrived at the knowledge upon which our conclusions are based, for such a consideration of methods will doubtless strengthen confidence in the conclusions.

146. A Determination Of The Elements Essential To The Construction Of The Human Body

Definite information as to the constructive elements of the tissues of the human body has been obtained by chemical analysis. It is a fair assumption that whatever is uniformly found present in a normally nourished organ or tissue is essential either to its construction or welfare. To be sure, the human body may retain certain substances in quantities that are not only unnecessary, but injurious, as, for instance, arsenic; but when a salt of potassium is found in the muscular tissue in practically constant proportions under all conditions of nourishment, it is fair to assume that normal muscles could not exist without it. Indeed, we know that the absence of certain elements would make impossible the presence in the human body of those compounds that are essential to its very existence.

147. Methods Of Ascertaining The Functions Of The Various Nutrients And The Needs Of The Human Body Under Varying Conditions

Our understanding of the functions of the food compounds and of the most efficient, and physiologically most economical, administration of dietaries under varying conditions is still incomplete. The exact knowledge we do possess is the result of many years of laborious investigation. We still lack much essential information concerning the special physiological relations and reactions of the individual compounds found in human food, but the general metabolism, or food exchange and use, in the animal organism is now sufficiently well understood to admit of many safe and important conclusions.

Children and adults who are living under varying conditions of age, environment, and activity consume individually a given amount of food daily. Certain questions arise. Is the food sufficient in quantity? Is it of the right kind from the physiological point of view; that is, are the nutrients of the right kind and in the right proportion? A general practical answer to these two questions might be found in an observation as to the health and physical status of the individual, which are the facts of ultimate importance. The laws of nutrition cannot be established by such general observations, however. Even changes in body weight are not a safe basis for concluding whether a given diet is meeting the nutritive demands of a given individual. A loss of fat may be replaced by a gain of water. It has been shown also1 "that a change from a diet poor in carbohydrates to one rich in1 carbohydrates is accompanied, by a considerable retention of water in the tissues of the body." In these ways we learn nothing of processes or how the physiological needs of the individual must be met, and we could not reason from one case of apparently successful nourishment to know how to meet the needs of another individual under entirely different conditions.