To one willing to accept the data as having possible significance there arises at once the question, How long can the body be maintained in nitrogen equilibrium on such relatively small quantities of proteid food? In other words, can experiments of this nature, extending over comparatively short periods of time, be safely accepted as a reliable means of measuring the proteid requirements of the body for indefinite periods? Suppose, says the critic, we grant that the body can maintain itself in nitrogen equilibrium for a week or two on a very small amount of proteid food, what proof have we that in the long run the body will be benefited thereby, or even able to exist in a condition of normal strength and vigor? In other words, is a low proteid diet, one that seems sufficient to maintain the body in nitrogen equilibrium, a wholly safe one to follow? May there not be other elements to be considered, aside from nitrogen equilibrium, which, if fully understood, would satisfactorily account for the customs of mankind, in which perhaps man’s instincts have been followed for the betterment of the race? It was with a view to learning more concerning these questions that five years ago the writer commenced systematic, experimental, work upon the nutrition of man, with special reference to his nitrogen requirements. The experiments and observations have been continued up to the present time, with many suggestive results, some of which will now be referred to.[60] [60] In presenting the general results of these experiments, the writer has drawn freely from his book, “Physiological Economy in Nutrition,” published by the Frederick A. Stokes Company, New York, 1904.