The formation of the body material and the repair of its wastes is the function of the proteids of foods. It has been found by careful experiment that a man at moderately hard muscular exertion requires .28 lb. of digestible proteids daily. The chief sources of our proteid foods are meats, fish, beans, etc. It has been as a proteid food that mushrooms have been most strongly recommended. Referring to Table I, it will be seen that nitrogen constituted 5.79 per cent. of the total dry substance of _Coprinus comatus_. This high nitrogen content, which is common to the mushrooms in general, was formerly taken to indicate a very unusual richness in proteid materials. It is now known, however, that there were several sources of error in this assumption. Much of the nitrogen is present in the form of non-proteid substances of a very low food value. Another and very considerable portion enters into the composition of a substance closely related to cellulose. A third source of error was the assumption that all the proteid material was digestible. It is now known that a very considerable portion is not digestible and hence not available as food. Thus, notwithstanding the 5.79 per cent. of nitrogen in _Coprinus comatus_, we find but .82 per cent. in the form of actually available (i. e., digestible) proteids, or approximately one-seventh of what was formerly supposed to be present. The digestibility of the proteids varies very greatly with the species. Mörner found the common field mushroom, _Agaricus campestris_, to have a larger amount of proteids available than any other species studied by him. Unfortunately, the digestibility of the American plant has not been tested. There is great need for further work along this line. Enough has been done, however, to demonstrate that mushrooms are no longer to be regarded as a food of the proteid class. The energy for the muscular exertion and heat is most economically derived from the foods in which the carbohydrates and fats predominate. The common way of comparing foods of the first two classes scientifically is to compare their heat-giving powers. The unit of measurement is termed a _calorie_. It represents the amount of heat required to raise a kilogram of water 1° Centigrade. (This is approximately the heat required to raise one pound of water 4° Fahrenheit.) A man at moderately hard muscular labor requires daily enough food to give about 3500 _calories_ of heat-units. The major part of this food may be most economically derived from the foods of the second class, any deficiency in the .28 lb. of digestible protein being made up by the addition of some food rich in this substance. In the following table the value of ten pounds of several food substances of the three classes has been worked out. Especial attention is called to the column headed "proteids" and to the last column where the number of heat-units which may be purchased for one cent at current market rates has been worked out.