METABOLISM 217 



The figures usually used for the average energy value of the 

 three foodstuffs are carbohydrates 4.1 large calories per gram, 

 proteins 4.1 and fats 9.3. It will be seen that the fats yield by 

 far the most heat per gram. (It will be recalled that a large 

 calorie is the amount of heat required to raise 1 kilo of water 

 through 1 C. of temperature, usually from 15 to 16 C. or 

 from to 1 C.) 



Accurate and extended studies have been made of the amount 

 of heat produced (and thus the amount of fuel required) by 

 individuals in health and in various diseases, at rest and during 

 exercise, waking and sleeping, during periods of mental ease 

 and of great mental exertion, at different ages from infancy to 

 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 isodynamic 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 



