The principles of metabolism have been sufficiently explained in the foregoing chapters to make it possible to understand the basis of a diet which shall be physiologically rational.

It has been seen that the average starvation metabolism of a vigorous man at light work and weighing 70 kilograms approximates 2240 calories, or 32 calories per kilogram. It is obvious that this quantity of energy must be contained in the daily food, and a little more to counterbalance the "specific dynamic" or heat-increasing power of the food-stuffs, if the individual is to be maintained in calorific equilibrium. It has been seen that when an average mixed diet is ingested the maintenance requirement is between 11.1 and 14.4 per cent, above the starvation minimum (p. 239). This would amount to from 2488 to 2562 calories, or from 35.5 to 36.6 calories per kilogram of body weight in the case of the individual just referred to.

Rubner1 is authority for the following table, which indicates the energy requirement of men of various weights while doing light work:

Since man through clothing shuts himself off from the reflex action of cold on the skin, the greatest factor which tends to increase his metabolism is mechanical work, and the total amount of calories required is here dependent on the kind and the amount of the work accomplished. The requirements in this regard have already been discussed.

1 Rubner: von Leyden's "Handbuch der Ernahrungstherapie," 1903, Bd. i, P. 153.

A point of great interest is that of the proper proportion in which the individual food-stuffs should be put together in making up a ration.

Voit defines a food as a well-tasting mixture of food-stuffs in proper quantity and in such a proportion as will least burden the organism. What is the proper proportion?

Voit1 gives the following ration for the use of an average laborer, such as a soldier in a garrison - that is, for a man at work from eight to ten hours a day: Protein, 118 grams; carbohydrates, 500 grams; fat, 56 grams. This diet contains 3055 calories.

Such a ration means the food actually ingested. It is also assumed that the food-stuffs are administered in a digestible form, and are therefore completely assimilable. It has already been pointed out in the Introductory Chapter that the feces contain no undigested protein when good food is given. It is, therefore, fallacious to deduct the nitrogen of the feces from the nitrogen of the ingesta in order to determine the amount of protein assimilated. Fecal nitrogen plus urinary nitrogen together represent the waste of assimilable protein nitrogen (see p. 47).

The allowance of 118 grams of protein has provoked much discussion. The original figures were obtained by Voit by averaging the protein metabolism of many laboring men. This requirement of protein was therefore obtained by the statistical method, which simply showed what the average laborer in habit consumed. For the same class of artisan the diet given by Rubner calls for 127 grams of protein; by At-water, 125 grams; and Lichtenfelt2 confirms Voit's average as being the quantity of protein taken by laborers in northern Italy.

1 Voit: "Physiologie des Stoffwechsels," 1881, p. 519.

2 Lichtenfelt: "Pfluger's Archiv," 1903, xcix, 1.

For men at hard labor, such as soldiers in the field, even higher quantities of protein are commended - by Voit, 145 grams; by Rubner, 165 grams; by Atwater, 150 grams. These figures again are based on statistics. Woods and Mansfield1 found that the average protein in the ration of fifty lumbermen is 164 grams.

In striking contrast to this Siven,2 at the age of thirty-one and a half years and weighing 65 kilograms, finds he can maintain himself in nitrogen equilibrium for a short period on a diet containing between 4 and 5 grams of nitrogen, or 25 to 31 grams of protein. In fact, in one experiment the food contained 4 grams of nitrogen, of which 2.4 grams only were in 15.4 grams of true protein and the balance in amino-acids and other nitrogenous non-protein matter of vegetable origin. Here nitrogen equilibrium was nearly attained, the nitrogen ingested being 4, and that excreted 4.28 grams. The food given, which was rich in carbohydrates, contained 2717 calories, or 43 calories per kilogram, and the total metabolism, as estimated by respiration experiments, indicated a heat production of 2082 or 32 calories per kilogram. Here was practically nitrogen equilibrium maintained at the minimum level, and a low total metabolism which was largely at the expense of carbohydrates.

It will be recalled that the quantity of nitrogen in the urine in the average fasting man who has been previously well nourished is 10 grams, a minimum which is reducible only by carbohydrate ingestion.

The experiments of Siven did not satisfy people that a low protein metabolism was compatible with continued health and strength. Munk3 and Rosenheim4 both found that dogs given a quantity of protein sufficient only to maintain nitrogen equilibrium gradually lost strength and became afflicted with digestive disturbances. These experiments fortified the idea of the benefits to be derived from a diet containing more protein than was necessary for the maintenance of nitrogen equilibrium - a luxus consumption. Rubner declared that a large protein allowance is the right of civilized man.

1 Woods and Mansfield: "Studies of the Food of Maine Lumbermen," U. S. Department of Agriculture, 1904, Bulletin 149.

2 Siven: "Skan. Archiv fur Physiologie," 1901, xi, 308. 3 Munk: "Archiv fur Physiologie," 1891, p. 338. 4 Rosenheim: Ibid., p. 341.

The tradition that a continued liberal allowance of protein in a diet is a prerequisite for the maintenance of bodily vigor has been dispelled by Chittenden1 and his co-workers, of whom Mendel is the most prominent.

Professor Chittenden had suffered from persistent rheumatism of the knee-joint, and determined on a course of dieting which should largely reduce the protein and the calorific intake. The rheumatic trouble disappeared, and minor troubles, such as "sick headaches" and "bilious attacks," no longer recurred periodically as before. "There was a greater appreciation of such food as was eaten; a keener appetite and more acute taste seemed to be developed, with a more thorough liking for simple foods." During the first eight months of the dieting there was a loss of 8 kilograms of body weight. Thereafter for nine months the body weight remained stationary. "Two months of the time were spent at an inland fishing resort, and during a part of this time a guide was dispensed with and the boat rowed by the writer frequently 6 to 10 miles in a forenoon, sometimes against head winds (without breakfast), and with much greater freedom from fatigue and muscular soreness than in previous years on a fuller dietary".