932 NUTRITION AND HEAT REGULATION. 



the non-proteins are not sufficient in quantity some of the energy 

 is obtained at the expense of the protein of the body, and there is 

 an increase in the nitrogen excretion. We may believe, in fact, 

 that the energy necessary for muscular work may be obtained from 

 any of the customary foodstuffs carbohydrates, fat, or proteins. 

 It seems probable that the sugar (glycogen) of the muscle is, so to 

 speak, the easiest source ; but, when the carbohydrates are deficient 

 or absent altogether in the diet, muscular exercise is accompanied 

 by an increase in the consumption of fats or proteins or both. 

 According to the view adopted in the preceding pages, it will be re- 

 membered that when protein-food is used as a source of energy 

 the nitrogen is split off in the tissues by the process of deaminiza- 

 tion of the amino-acids. According to this view, therefore, the 

 working muscle cells obtain their energy always by oxidation of 

 non-nitrogenous material, although a portion of this material may 

 have been derived ultimately from the protein of the food. The 

 Voit theory is correct to the extent that on an abundant non-pro- 

 tein diet much muscular work may be done without any increase 

 in the consumption of the organized protein tissue. The muscle 

 is a protein machine for the accomplishment of work, but in the 

 performance of moderate work there is apparently no greater 

 wear and tear of the machinery, no greater tissue waste, than under 

 resting conditions. If, however, the muscular work is excessive, 

 the tissue waste may be increased. Argutinsky found an in- 

 creased nitrogen elimination lasting two or three days after the 

 cessation of the work. It is probable that this result indicates a 

 greater waste of the protein apparatus itself, and this idea is borne 

 out by the fact that under similar conditions other observers have 

 detected an increase in the creatinin and uric acid excretion, 

 nitrogenous wastes that are derived from the tissue protein of 

 muscle. The effect of muscular work on the carbon excretion, car- 

 bon dioxid, is, of course, marked and invariable. Some extra ma- 

 terial must be oxidized to furnish the energy, and since this material 

 is usually sugar, or sugar and fat, or the non-nitrogenous portion 

 of the protein of the diet, the effect, so far as the excretions are con- 

 cerned, will be most manifest in the amount of carbon dioxid 

 given off. Pettenkofer and Voit found that the carbon dioxid 

 eliminated by a man during a day of work was nearly double that 

 excreted during a day of rest. Along with this rise in the carbon 

 dioxid excretion there is a corresponding increase in the absorption 

 of oxygen. These results are well illustrated in the following 

 table, which shows the effect of posture and of severe muscular 

 work upon the hourly excretion of carbon dioxid and absorption of 

 oxygen (Benedict and Carpenter).* 



* Carnegie Institution of Washington, No. 126, 1910. 



