218 PHYSIOLOGICAL CHEMISTRY 



requirement sinks to perhaps 1,700 calories. Physical exercise 

 and exposure to cold greatly increase the energy requirement, 

 which may run up to 4,000 or even 5,000 calories. The extreme 

 limit recorded is the case of an endurance bicycle rider who used 

 up 10,000 calories in a day. 



An interesting fact is that the mere taking of food will mate- 

 rially increase the heat production in the body. It was suggested 

 that this was due to the work of the digestive glands and organs, 

 but this has been shown to be incorrect ; the taking of salts which 

 cause intense muscular activity of the intestine resulted in no 

 such increase. This effect of food substances is spoken of as 

 the Specific Dynamic action of the foods. Proteins cause the 

 greatest rise in heat production. The effect in the case of pro- 

 teins is believed to be due to a stimulation of the cells of the 

 tissues to greater activity, the stimulation being produced by the 

 decomposition fragments of protein in the blood. In the case 

 of the carbohydrates and probably also of the fats, the effect is 

 believed to be due to the "mass action" of the fragments of 

 decomposition and not to a direct stimulating action. Since 

 proteins produce the greatest increase in heat production, they 

 should be eaten sparingly in hot weather. 



Curiously enough, increased mental activity does not increase 

 the heat produced by the body. This might be interpreted to 

 mean that mental activity uses up no fuel, or an inappreciable 

 amount, but it is more nearly accurate to state that great mental 

 activity such as studying for an examination produces no more 

 heat than the mental activity of our ordinary and most indolent 

 mental processes. 



During sleep metabolism is low, but this is due probably to 

 lessened body activity rather than to any inherent differences 

 of metabolism. 



The variations of heat production in disease have been care- 

 fully studied by Benedict, Lusk, du Bois and others. In typhoid 

 and exopthalmic goitre there is a great increase. In diabetes 

 the increase is slight. 



As a basis for comparison, the heat production of the body at 



