144 METABOLISM AND NUTRITION 



With males we see that the excretion of carbon dioxide is greater between 

 the ages of fourteen and nineteen than in older or younger individuals of the 

 same sex. This agrees very well with Key's observation on the growth of boys, 

 namely that beginning with the fourteenth year the increase of the body in 

 length and weight takes place much more rapidly than during the years imme- 

 diately preceding (nine to thirteen). This period of rapid growth continues 

 for four years (cf. Chapter XXVI, second section). 



To judge by the elimination of C0 2 , a boy from nine to thirteen, therefore, 

 would need almost as much food as a man resting, and boys between fourteen 

 and nineteen still more. We must not overlook the fact, however, that the 

 calorific value of the C0 2 is very different according as it has its origin in the 

 metabolism of fat, carbohydrates or proteid. Since in the above table the in- 

 dividuals on whom the experiments were made belonged to the same class 

 of society, and so far as the diet, etc., were concerned lived on the whole on 

 the same plane, it may be assumed with great probability that the average 

 composition of their diet, and consequently the share of the different foodstuffs 

 in the formation of C0 2 , was about the same. 



With females the C0 2 -elimination does not show the significant rise which 

 appears in boys between fourteen and nineteen. From the eleventh year on 

 but slight differences due to age make their appearance : in an eleven-year-old 

 girl the C0 2 -excretion was 26 g., in a woman of thirty, 29 g. We might say, 

 therefore, that the requirements of a girl of eleven are just as great as those 

 of an adult woman at rest. 



Comparison of the CO 2 -output of males and females of the same weight and 

 age shows that during the years of growth it is considerably greater with the 

 former than with the latter, and that the ratio of female metabolism to male 

 metabolism estimated per kilogram of body weight is about 100:140. In men 

 and women who have already passed the period of growth this difference gradu- 

 ally diminishes, and as old age comes on disappears altogether. 



The figures of this table differ considerably from those given by Magnus- 

 Levy and Falk in the table on page 118. The reason is that the subjects of their 

 experiments had not eaten recently and were in absolute muscular rest, while 

 the results brought together in the table now under consideration were obtained 

 upon individuals in a sitting posture shortly after a meal. On this account 

 Magnus-Levy and Falk found no difference in the C0,-excretion by males and 

 females. The difference which we have noted above is, in all probability, traceable 

 to a greater tonus in boys' muscles than in girls'. 



By way of comparison with the direct data on the metabolism of the grow- 

 ing body we may add also the standard figures which Atwater uses in appor- 

 tioning the diet of a family to its different members. Taking the food require- 

 ments of the father as 1, the requirements of the others would be : 



Of the mother 0.8 ; 



" sons, 14-17 years 0.8 ; 



" daughters, 14-17 years 0.7 ; 



children, 10-13 years 0.6; 



6-9 years 0.5; 



2-5 years 0.4 ; 



" " under 2 years 0.3. 



