=The Relative Value of Foods.=—Dried proteins contain about 15 per cent. nitrogen, 54 per cent. carbon, 7 per cent. hydrogen, 22 per cent. oxygen, a little sulphur, and frequently some phosphorus. A large proportion of their carbon and hydrogen is available for combustion. Fats contain 75 per cent. of carbon, and a considerable quantity of hydrogen available for combustion; carbohydrates, 40 per cent. of carbon, with hydrogen and oxygen in the proportions in which they occur in water. If 1 gramme of protein is oxidized to the condition of urea, carbonic acid, and water, sufficient heat is liberated to raise the temperature of 4,100 grammes of water 1 degree centigrade. Its calorific value is therefore expressed as 4,100 calories, the unit of measurement—a calorie—being the amount of heat needed to raise 1 gramme of water 1°. The calorific value of 1 gramme of fat is 9,300 calories; of 1 gramme of starch, 4,100 calories. Thus, the energy potential in protein and in starch is the same; that in fat more than twice as great as that in either of the other foods. =A Normal Diet.=—Nitrogenous equilibrium and body-weight can be maintained and work done on diets which vary widely in percentage composition. This is a question which we shall consider at greater length later on. In the meantime, for the sake of illustration, it is necessary to formulate a diet which is fairly representative of the selection of foods made by a man of average weight—say 70 kilogrammes (145 pounds)—who desires to do a moderate day’s work in comfort. It has been found to amount to about 100 grammes of protein, 100 grammes of fat, 240 grammes of carbohydrate, all measured dry and as pure foods. If the several elements of such a diet be multiplied by the figures which represent their calorific value, it will be found that the man is supplied with 2,324,000 calories. The illustration that we have chosen is the diet of a professional man who is not engaged in hard physical work. The pure foods would be found to the amounts stated in 17 ounces lean meat, 4 ounces butter, and 17 ounces bread. The day’s diet would, of course, be much more varied than this, but it is simpler to express it in these terms. Such a diet would hardly answer the requirements of a man doing hard muscular work. Experience shows that he would expect to receive a more liberal supply of energy, and that to obtain it he would increase slightly his allowance of proteins, and very considerably increase the quantity of carbohydrates that he consumed. The diet of European workmen is remarkably constant in the relative amounts of its several constituents, no matter what their nationality or the exact form of their work may be: Proteins, about 135 grammes; fats, 80 grammes; carbohydrates, 500 to 700 grammes—giving a supply of energy equal to 3,500 to 4,000 kilo-calories.