The studies of McCollum and Simmonds have introduced into the philosophy of nutrition the idea that the word "normal" is too indefinite a term, and less satisfactory than "optimal" to designate the standard of excellence toward which we should strive in respect to physical development. They emphasize that our standards of what constitutes normality are based upon common observation rather than upon the best possible achievement as illustrated by the best specimens in the population. These investigators also pointed out that it is unsafe to judge by the appearance or feelings of a man or animal under special conditions during a relatively small fraction of life, especially that part in which recuperative power and endurance are greatest, that a dietary regimen which has proven unsafe for the promotion of vigor and health in animals, is safe for man. Physical deterioration may, and generally is unnoticed or not admitted until it is pronounced. It is important to appreciate that in the aggregate, it is borderline states of malnutrition, characterized by a condition of nutritional instability, that is the greatest menace to the individual when his health and efficiency over the life period is considered.

With this attitude toward the subject of our discussion, we may now turn to some of the more technical phases of nutrition investigations, the results of which have served as a basis of practical conclusions of great value for the health of man.