CHAPTER IX. 



WORK. 

 The Influence of Work on the Output of Nitrogen. 



As the endogenous output of nitrogen during rest is so small it 

 might naturally be expected that it would be increased when the 

 conditions were altered, i.e. when more active metabolism took place, 

 as during work. Practically, however, every investigator of this 

 problem of the extent of metabolism during exercise has found that 

 work leads to little change in the output of nitrogen in the urine, 

 provided always that the supply of food, particularly of carbohydrate 

 and of oxygen, be sufficient. The source of energy during work has 

 been the subject of investigation for a great many years, and natur- 

 ally the most divergent views are found. Thus, Voit and his school 

 looked on fat as the most important source of energy ; Chauveau 

 and Seegen considered that carbohydrate was the important material ; 

 Pfliiger at first held that the sole source of energy lay in the protein, 

 but later he modified the idea to some extent. Others held that all 

 three foods played a part in the supply of energy. 



No one has yet been able to show clearly that during work there 

 is any great utilization of protein, as evidenced by a marked increase 

 in the output of nitrogenous waste products in the urine. "v Thus, Voit 

 (VII) found that, if a fasting animal were exercised, there was only 

 a small increase in the output of nitrogen. In a thin young animal 

 the increase varied from 8 to 16 per cent, of the total amount, and in 

 an old fat animal, after eight hours' hard continuous work, the increase 

 was from I to 8 per cent, of the total amount of nitrogen excreted. 

 The differences noticed between the effect of exercise taken on a full, 

 or an empty, stomach were only slight. Voit and Pettenkofer (VII) 

 confirmed this negative effect on the nitrogen output in work both 

 during hunger, and feeding, periods in a man. Voit was inclined to 

 ascribe these slight rises after work to the complete (Utilization of the 

 nitrogen-free foodstuffs which was followed by the burning up of some 

 of the protein tissue, owing to the lack of fuel. Oppenheim (307), who 



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