The researches in the field of nutrition have a greater value in preventive medicine in relation to raising the vitality of mankind, with all that this implies, than they have in the prevention of the occurrence of the deficiency diseases. This fact has never been sufficiently appreciated. In the aggregate, by far the most important effect of faulty nutrition in man is the result of errors of one kind or another which are not sufficiently grave to cause prompt and spectacular failure. It is the gradual operation of more or less constant, but unperceived causes, rather than of great and marked exposures of an accidental nature which in nearly all cases are responsible for undermining the health of the individual. Of these causes, it now seems certain, the consumption of an improperly constituted diet is one of the most important. It is the cause of inferiority in physical development, instability of the nervous system; lack of recuperative power and endurance and consequent cumulative fatigue, and lack of resistance to infections such as tuberculosis and other types where specific immunity is not easily developed by the body. In addition to these the rate of development of senile characters and consequently the length of the span of life, are greatly influenced by the type of diet employed. It is important for us to inquire how far such defective diets as result from unwise selection of ordinary foods, can influence the life history of man and animals.