CHAPTER XX 

 THE FACTORS WHICH MODIFY METABOLISM 



THE circumstances which radically affect the quantity 

 and kind of decomposition going on in the body must al- 

 ready be evident. There can be no hesitation in placing 

 first among these conditions muscular activity. Minimal 

 metabolism attends the most nearly complete state of 

 rest which can be secured. It is somewhat lower during 

 sleep than during a like period of lying awake, doubtless 

 because the waking subject cannot so successfully abolish 

 muscular contraction. When one is sleeping metabolism 

 obviously continues in certain of the breathing muscles 

 and the beating heart. We cannot doubt that it proceeds 

 also in the glands, the muscular coats of the alimentary 

 canal, the nerve-centers, and many other localities. The 

 lowest rate at which the metabolism of a healthy adult is 

 likely to go on during sleep is in the vicinity of 50 Cal. per 

 hour. 



Very moderate activity suffices to double this rate of heat 

 production. More vigorous exercise raises the figure in 

 many cases to 200 or 300 Cal. per hour, while we have records 

 of as much as 600 Cal. per hour. This strikingly high level 

 was reached by a professional bicycle rider exerting himself 

 to the utmost upon a stationary bicycle inside a calor- 

 imeter. The total for a day observed upon an athlete 

 working to the limit of endurance may reach 9000 Cal. 

 Changes so extreme as these could hardly fail to attract 

 early attention. Lavoisier in the eighteenth century 

 noticed that exercise led to increased consumption of oxy- 

 gen. Everyday experience of the heating and the "wind- 

 ing" effect of activity would lead to the clear impression 

 of heightened metabolism. 



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