We have seen that the common experience of mankind and the evidence of scientific inquiry agree that a sufficient amount of food must be taken daily to yield from 2,500 to 3,000 calories. When we come to inquire of what constituents this food should consist, we find a general agreement upon fundamental points, but a great deal of difference of opinion upon others. It is established that the dietary of man should include all three food-stuffs, protein, fat, and carbo-hydrate. The protein is essen-tial: no other material can supply the loss involved in the wear and tear of living organs. No other food-stuff can entirely supply the needs of an animal as protein can those of the carnivora. In the case of man, a healthy existence cannot be supported upon protein without carbo-hydrate, and it is a great advantage to him to have fat in his diet as well, since fat gives a greater proportion of energy, weight for weight, than carbo-hydrate or protein. We have seen that protein furnishes material for the metabolism of structure, whilst carbo-hydrate and fat, and any protein in excess of that required for structure, furnish energy for the metabolism of function, or put in another way, fuel value to supply heat and work. Two points arise for consideration, first, what is the minimum amount of protein which is essential for existence, and secondly, what is the amount which is desirable in order to maintain the body in the highest degree of efficiency? The first question we can answer upon the evidence before us, but there is considerable disagreement on the second. Provided that a certain minimum of protein be supplied, and sufficient caloric value, an active life can be supported upon very varying proportions of the three elements. This fact must be clearly borne in mind. Any experiments showing that men can live and work for long periods upon this diet or upon that, provided that the constituents satisfy the above fundamental condition, show us nothing new. Men have existed in the past, in the vicissitudes of wealth and poverty, freedom and captivity, upon dietaries as varied in both quantity and quality as will ever be designed by experimentalists. The main object of our inquiry must therefore be not to determine upon how much or how little a man can live, but what are the proportions of the food-stuffs upon which he can live with the greatest efficiency and economy.