main purposes in the animal economy: (1) They supply building and repair materials, and (2) they furnish energy for all the physiological activities. Let us consider the latter purpose first. Physiologists have demonstrated by numerous experiments that all the energies manifested in the body finally leave the body as heat which can be measured by an instrument known as the calorimeter. The heat unit employed in these calculations is known as a calorie, which is the amount of heat necessary to raise one kilogram of water 1° Centigrade. The quantity of heat measured agrees exactly with the quantity which may be calculated from the known amount of oxidation of foodstuffs which has taken place in the body. When muscular work is done in a calorimeter and the work is all made to take the form of heat, the increased heat production is again what it should be as judged by the increased oxidation. Similarly, the energy value of foods is determined either by analysis and computation or by burning it in oxygen. Given then foods capable of producing a certain quantity of heat, it is a fairly easy task to compute the amount of each which would be necessary to furnish the energy requirements of the body under any given set of conditions.
An engineer who wishes to supply a certain amount of power must know the heat value of certain kinds of fuel and the waste from each. From these he reckons the net cost of his power. Any one who cares to do so can make the same sort of a computation for his body. If the engineer pays $7 for a ton of coal, he sees to it that he gets $7 worth of heat. Why is it not just as reasonable when a person pays a certain price for food to expect a certain amount of food value? To demand the worth of one's money in heat units when the fuel in the house is under consideration is a plain proposition, and when housewives generally understand food values it will be a plain proposition in respect to fuel for the body. A properly educated public opinion will demand from manufacturers such information in regard to the food on which its energies depend.
Let us see if this is not a practical suggestion, and whether, after all, it may not prove a simple solution to a supposedly difficult problem. If our foods were all simple substances like sugar or olive oil, and if the energy content of the food were the only one of which we need take any account, the problem would be just as easy as calculating the yield of energy in horse-power from a ton of coal. It is because our foods are mixtures of various foodstuffs, each having a different fuel value and a different functional value, that the matter requires some study.
The simplest method yet devised for keeping account of the energy supply in one's diet is that devised by Professor Irving Fisher, of Yale University. The idea underlying this method is to do away with intricate calculation by familiarizing one's self with the amount of each
