METABOLISM UNDER VARYING CONDITIONS 343 



Work. During work the metabolism is increased. This is due 

 to the increased activity of the muscles. Investigation shows that 

 this is mainly at the expense of the fats and carbohydrate, and but 

 little at the expense of the proteins, unless there be a deficit of the 

 two first. The respiratory exchange is greatly increased by muscular 

 work (see p. 319). 



A subject pedalled a b cycle sixteen hours out of twenty-four. 

 The whole energy expended = 9,981 calories. The energy of the food 

 taken = 5,138 calories. The energy taken out of the body substance 

 = 4,843 calories. The energy derived from tissue protein only = 478 

 calories. As the man was an athlete in training, his muscles were not 

 overdone, and thus the protein metabolism was scarcely increased. The 

 experiment shows how body fat may be taken off by hard exercise. 



When no muscular work is done, as during the first hours of 

 sound sleep, the metabolism of the body markedly decreases; during 

 the waking hours it again increases. But if tb.3 individual keeps at 

 rest and protected from cold, there is no increased metabolism during 

 these hours. ' 



External Temperature. A lowering of the external temperature 

 excites to more muscular movement, voluntary and involuntary, 

 and thereby increases metabolism. Shivering helps to keep up the 

 temperature. Exposure to cold wind may double the respiratory 

 exchange, even if the individual sits quiet in a chair and does not shiver. 



There ir a relation between the cooling power of the atmosphere, 

 the skin temperature, and the rate of metabolism. Thus the appetite 

 is increased at a bracing place. 



The raising of the body temperature increases the metabolism. 



Feeding. The taking in of food raises the metabolism, partly owing 

 to the mechanical work involved in digestive processes, partly owing 

 to the chemical processes. With some foods this may be very great 

 as much as 50 per cent, of the energy value of the food taken in. For 

 instance, with a horse chewing hay, 48 per cent, of the energy value 

 is used up in the work of mastication and digestion, whereas but 

 19-7 per cent, is so used in the case of oats. It has been calculated, 

 for the caloric value of the food taken there is expended in heat pro- 

 duction during digestion and assimilation- 

 Fats . . . . . . . . . . about 2 per cent. 



Starches . . . . . . . . . . ,, 9 ,, 



Proteins . . . . . . . . . . ,,17 ,, 



Proteins, having so high " a specific dynamic energy,' 1 are heating, 

 thus more protein is consumed by man in cold, and less in tropical 

 climates. In the adaptation of an Englishman to a tropical climate 

 diet is of great importance. On a diet of rice and bananas monkeys 

 successfully withstand exposure for hours to the tropical sun. 



