194 PHYSIOLOGICAL CHEMISTRY 



old age, and in the two sexes. It will be possible here only to 

 summarize briefly some of these important findings. 



First, the different foodstuffs are isodynamie in the body, that 

 is, the body can employ fats, carbohydrates or proteins as fuel 

 interchangeably, and with no loss of energy, each foodstuff 

 furnishes its total theoretical amount of energy when it is oxi- 

 dized in the body. 



The total amount of energy required by an individual varies 

 with his age, state of health, body weight, sex, and degree of 

 body activity. In general, the energy exchange is higher in 

 small animals than in larger ones, for there is more surface per 

 unit of weight in small animals, and thus the loss of heat is 

 greater. Metabolism in children is higher than in adults, partly 

 for the foregoing reason, and possibly also because there is 

 greater tissue activity. In newborn infants during the first few 

 days of life, metabolism is low. In a fat man the energy ex- 

 change per kilo body weight is lower than in a thin man, as the 

 excess weight is due to inert fat deposits which take little part 

 in active metabolism. Metabolism is lower per kilo body weight 

 in women than in men. 



Increase in body activity, of course, increases the energy 

 exchange. More fuel is burned, and more heat produced. The 

 average city adult man requires from 2,500 to 3,000 calories per 

 day. If such a person remains in bed and inactive, his energy 

 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 



