Professor Chittenden also points out the common blunder which is made in assuming that persons who are doing hard work need an additional amount of proteid substance. One commonly hears the phrase that laborers and athletes can eat meat in large quantities, and “work it off.” As we have seen, one can “work off” sugars and starches and fats completely; but one cannot “work off” proteid completely. Professor Chittenden is now recognized as the leading authority of the world upon this particular question; and he sets forth clearly in his book the fact that the quantity of proteid needed is not increased by muscular activity. One may work as hard as he pleases, but his body will use no more nitrogen, save only in the case where a sufficiency of other food elements is not supplied. Only as a last resort will the system undertake the labor of burning up proteid to make energy. HOW MUCH SHOULD WE EAT When foodstuffs are taken into the body, digested, assimilated, and used up, they produce the same amount of heat and other forms of energy as if burned outside of the body; and hence it follows that the number of calories, or units of heat, represented in a given foodstuff, is taken by scientists as a common measure of its food value. A calory is a heat unit, which has been adopted as a means of estimating the nutritive value of foodstuffs. It represents the amount of energy required to raise the temperature of four pounds of water one degree Fahrenheit. The number of calories contained in food is obtained by burning the food and measuring the heat produced by means of a calorimeter. It has been calculated that the normal, average person needs from one hundred and sixty to two hundred and forty calories of proteids every day, in order to build blood and tissues. He needs daily from five hundred to nine hundred calories of fats, which supply heat. He needs of carbohydrates, which are the starches and sugars, and which the body uses to produce energy for work and heat, from one thousand to one thousand four hundred calories daily. It is declared by Chittenden and Kellogg, whose work has overset the old notions, that the total number of calories, or food units, should rarely exceed two thousand. Two thousand calories are furnished respectively by twenty-eight ounces of bread, or ninety-six ounces of milk, or sixty-two ounces of potatoes, or nine ounces of butter. One quarter of each of these, or any other fractions which together equal unity, will make up a ration containing two thousand calories.