As has been stated several times, the proteid foodstuffs are the more important, since proteid matter is essential to animal life. Man must have a certain amount of proteid food to maintain the body in a condition of strength and vigor. The other essential is that the daily food furnish sufficient energy to meet the needs of the body for heat and power. This means that in addition to proteid, which primarily serves a particular purpose, there must be enough non-nitrogenous food (either carbohydrate or fat or both) to provide the requisite fuel for oxidation or combustion to meet the demands of the body for heat and for work; both of which are subject to great variation owing to differences in the temperature of the surrounding air, and especially because of variations in the degree of bodily activity. The energy which a given foodstuff will yield can be ascertained by laboratory experiment, in which a definite weight of the substance is burned or oxidized in a calorimetric bomb under conditions where the exact amount of heat liberated can be accurately measured. The fuel, or energy, value so obtained is expressed in calories or heat units. A calorie may be defined as the amount of heat required to raise 1 gram of water 1° C., or, to be more exact, the amount of heat required to raise 1 gram of water from 15° to 16° C. This unit is usually spoken of as the small calorie, to distinguish it from the large calorie, which represents the amount of heat required to raise 1 kilogram of water 1° C. Hence, the large calorie is equal to one thousand small calories. When burned in a calorimeter, 1 gram of carbohydrate yields on an average 4100 gram-degree units of heat, or small calories; 1 gram of fat yields 9300 small calories. Both of these non-nitrogenous foods burn or oxidize to the same products--viz., carbon dioxide and water--when utilized in the body as when burned in the calorimeter; hence, the figures given represent the physiological heat of combustion, per gram, of the two classes of foodstuffs. Obviously, the fuel values of different foods belonging to the same group or class will show slight variation, but the above figures represent average values.