Food eaten represents potential chemical energy, the source of bodily energy. To determine the amount of potential energy in food eaten it is necessary to know how much food is consumed and the potential energy of food-stuffs. The calorie is the unit that has been fixed upon to express the energy stored in food. A calorie is the amount of heat required to raise one gram of water 1° C. The most convenient way in which to estimate the value of food-stuffs is to determine how many calories a given weight will furnish. It has been established that one gram of dry protein will furnish 4.1 calories, the same quantity of carbohydrate 4.1 calories, and the same quantity of fat 9.3 calories. With these facts known, it is easy to determine the calories that any given food-stuff will furnish, provided the percentages of protein, fat, and carbohydrate that it contains are known. These percentages must be multiplied by the figures just given and their results added in order to ascertain the number of calories in 100 grams of the food.

As has already been shown, a certain amount of protein is necessary to sustain life. It is not sufficient, therefore, to say that a food to be able to maintain life and strength can furnish a given number of calories. Any number of calories can be obtained from fat, but fat will not maintain life and promote growth of cells. Food must be estimated in terms of protein required and calories required. A large proportion of the latter can be obtained from variable amounts of fats and carbohydrates. The number of calories required by an individual man will necessarily depend upon his weight and the amount of energy which he expends in work or muscular exercise. In establishing a standard, the weight fixed upon by various investigations has been approximately 150 pounds. The number of calories required by a man of this size when leading a sedentary life has been determined by four methods:

1. By observing the average amount of food consumed by many men in different countries and under varying conditions.

2 By observing the amount of oxygen consumed.

3. By determining the balance of intake and output.

4. By direct measurement of heat given off by the body. All of these methods have given approximately the same results. A man at rest requires 2000 calories and one leading a sedentary life needs 2300 calories daily.

The amount of protein required cannot be so accurately determined. This has been attempted generally by observing the amount used in various places and by different people. Voit in Germany fixed upon 118 grams of protein as the standard. Playfair in England fixed upon 119. Gautier of France fixed upon 107 and Atwater in this country fixed upon 100 grams for a man of sedentary habit, 125 for one at moderate work, 150 for one at hard work and 90 for one completely at rest.

Chittenden objects to the deduction of a standard from these facts; for such a standard, he says, is based upon evidences of self indulgence not upon the needs of an individual or the most profitable use of food. To determine the amount of protein needed he experimented on many individuals, gradually reducing the amount of protein in their food to as low a point as was compatible with good health and mental and physical strength but permitting the calories required for the production of energy. In this way he found that approximately 50 grams were needed. This is the minimum which insures health and strength. Therefore Chittenden fixes as a standard 60 grams. He believes that much larger amounts overtax the organs of elimination and lead to disease. It must be remembered, however, that all the organs of the human body are capable of many times the amount of work required of them to maintain life and health. Nevertheless it is to-day generally admitted that more protein is ordinarily consumed than is needed and often more than is good for one. Most recent writers have, therefore, been inclined to compromise between the standard fixed upon by observing habits of eating and that fixed upon by Chittenden by experiment and have named 70 or 75 grams of protein as probably the safest standard.

It is interesting to compare these standards of 75 grams of protein and 2300 calories with the dietaries in the tables on pages 62 to 66.