The following experiment not only indicates the fully proved point that muscular work does not increase protein metabolism, but it also shows that the character of the protein metabolism is unchanged by muscular activity. Shaffer2 has given a man a diet which was free from purins and which contained only 5.9 grams of nitrogen. The individual spent the greater part of six days in bed as a rest period (I). He then occupied himself for five days with laboratory work, which gave a normal period (II). During a final period (III) of four days he worked in the laboratory and performed in addition such mechanical work as that of walking 10 miles. The average of the analyses of the unnes of the three periods are given below:

1 Zuntz and Schumburg: "Physiologie des Marsches," 1901. 2 Shaffer: "American Journal of Physiology," 1908, xxii, 445.

Unchanged Character Of The Urine After Muscular Work

Shaffer concludes that if sufficient food be allowed, an increase or decrease of muscular activity has no effect on protein metabolism as indicated by the various quantities of nitrogenous end-products which appear in the urine. Shaffer agrees with Van Hoogenhuyze and Verploegh4 that with adequate nourishment the creatinin elimination is unaffected by muscular work.

Kocher2 states that doubling the heat production of the day as brought about by walking 60 kilometers (37.5 miles), i. e., from Munich to the Starnberger See and back, has little or no influence upon the protein metabolism of men, whether the diet consists of starch, sugar and cream, or of meat and fat without carbohydrates.

Bornstein3 reports continual retention of ingested protein during seventeen days' work, at a time when only protein was administered. The quantity of protein given was large, containing 19.96 grams of N, and the daily work accomplished was moderate, being 17,000 kilogrammeters. The nitrogen retention amounted to 1475 grams daily, or an addition of 800 grams of "flesh" to the body in seventeen days.

Loewy4 reaches the same conclusion that long-continued muscular exercise favors protein retention. This suggests the basis of muscular hypertrophy due to physical exercise.

1 Van Hoogenhuyze and Verploegh: "Zeitschrift fur physiologische Chemie," 1905, xlvi, 415.

2 Kocher: "Deutsches Archiv fur klinische Medizin," 1914, cxv, 82.

3 Bornstein: "Pfluger's Archiv," 1901, lxxxiii, 540.

4 Loewy: "Archiv fur Physiologie," 1901, p. 299.

Large protein ingestion, however, is not apparently essential to the full maintenance of physical power. This has been shown by Chittenden,1 who maintained soldiers and athletes in physical training for months at a time on diets containing between 7 and 10 grams of nitrogen, or about half what the average man takes if the question be left to his taste (see p. 338).

It is evident that the power to accomplish muscular work is not usually derived from protein metabolism, but from the combustion of the non-nitrogenous sugar and fat.

Therefore, physical exercise requiring fat consumption without concomitant destruction of protein must be of the greatest value in the treatment of obesity.

The problem at once arises: What is the relative value of fats and carbohydrates as fuel for the production of mechanical energy by the body?

Zuntz,2 from experiments made by Heineman, calculates that when carbohydrates predominate in a man's diet an amount of energy above the resting requirement is liberated which equals 9.33 calories for every kilogrammeter of work accomplished, whereas, when fat is given, 10.37 calories are liberated in the performance of the same amount and the same kind of work. The work was done by turning the wheel of an ergostat. Since one kilogrammeter is the mechanical equivalent of 2.35 calories, it is evident that 25 per cent, of the total excess of energy developed by work is convertible into mechanical effect, the balance being dissipated as heat. Similar experiments made by Zuntz on himself showed that 9.39 and 9.33 calories of metabolism were liberated on a fat diet, 10.37 and 10.41 on a carbohydrate diet, when one kilogrammeter of work was accomplished.

1 Chittenden: "Physiological Economy in Nutrition," 1004. 2Zuntz: "Pfluger's Archiv," 1900, lxxxiii, 557.

There seems to be little difference in the efficacy of the body as a machine, whether fat or carbohydrates are used as fuel.

Heineman1 remarks that Chauveau's idea that fat must be first converted into sugar before being available for mechanical work can scarcely be valid, for such a conversion of fat carbon into sugar would entail a minimum loss of 29 per cent, of the energy available for mechanical work.

Atwater and Benedict2 thought that they had confirmed these results, although, unfortunately, the diets provided were not strictly fat-protein and carbohydrate-protein, but were really mixed diets.

Thus J. C. W., during two periods of twenty-two days each, ingested day by day diets which produced the following metabolism as calculated from the body's excreta:

The average of work accomplished and body heat evolved each day, as measured in the Atwater calorimeter, were as follows:

Work And Metabolism As Directly Measured

The work was done on a stationary bicycle. It is evident that the work could not have been at the expense of protein metabolism; but it is also plain that the work could have been derived from carbohydrate combustion, even in the "fat" diet of Period II.

1 Heineman: "Pfluger's Archiv," 1900, lxxxiii, p. 476.

2 Atwater and Benedict: "Experiments on the Metabolism of Matter and Energy in the Human Body," 1903, U. S. Dept. of Agriculture, Bulletin 136.