That this energy-giving quality of foods is the most important function which they serve is apparent at once when we discover that at least four fifths of the dry weight of our food serves no other purpose than that of giving heat. The heat comes from the oxidation of our food and since the temperature of the air about us is nearly always lower—in winter very much lower—than blood heat, we are practically always losing heat by radiation and conduction. Merely to keep up the body temperature to 98° F., the temperature at or near which the living substance best performs its functions in every animal, is the purpose of the great mass of fuel which we are obliged to "supply" every day of our lives. To furnish the energy for the muscular work which most of us do requires relatively little and, strangely enough, to supply brain energy—intellectual energy, alertness and other purely psychic qualities—apparently requires no energy at all in the sense in which we have been using that term thus far. Experiments have been conducted on college
