J74 HUMAN PHYSIOLOGY 



more food than during rest. For work the rational diet is, 

 proteids 130 g, fat 100 g, carbohydrate 500 g. In this diet 

 the proteids are also increased because a man doing hard 

 work has a better developed muscular system and therefore 

 a greater demand for proteids than a resting or slightly 

 active persor. Hence the proper diet of a working man 

 serves not only to replace material consumed but, as the 

 power of work is proportional to the amount of muscle, to 

 increase the amount of flesh. 



(b) Effect of the work of digestion. In considering the 

 activity of the body, the material and energy used in the 

 processes of digestion and absorption must not be omitted. 

 Under digestive work we include the activity of the glands, 

 the movements of the alimentary canal, and the activity 

 which the epithelial cells of the intestine exhibit in taking 

 up material from the intestine and transferring it to the blood 

 or lymph. But it is impossible at present to say how much 

 of the increased metabolism observed after the taking up of 

 food is due to this increased activity and how much is due 

 to the increased supply of material for combustion. It is 

 nevertheless beyond doubt that the digestive work causes a 

 considerable increase in metabolism, and it has even been 

 asserted that the difference between the metabolism of a 

 fasting and that of a fed organism is entirely due to this 

 increase in the activity of the alimentary canal. But it is 

 supposed that the energy for this activity is not supplied by 

 fats or carbohydrates but by proteids, for during digestion 

 the nitrogenous excretion in the urine is greatest; hence 

 more proteids are consumed at this time. 



(c) Effect of loss of body heat. The human body main- 

 tains its own temperature independently of the external tem- 

 perature. This body temperature is maintained at its proper 

 height by continual combustion (see Chapter XIII). The 

 body continuously loses heat which must be replaced by the 

 combustion of new food material. The amount of heat 

 given off depends upon the external temperature ; the lower 

 this is, the more heat is lost by the body and the more oxi- 



