THE NORMAL DIET OF MAN 735 



The processes of growth are attended moreover with an active meta- 

 bolism, so that the metabolism is more energetic in the young animal 

 than in the adult, even if we take into account the relative surface in 

 the two cases. In the following Table is given a number of observa- 

 tions showing the output of carbon dioxide per square metre body 

 surface at different ages in boys and men : 



EFFECT OF AGE ON METABOLISM (MAN) 



Between the ages of fourteen and nineteen years in boys the 

 carbon dioxide output is absolutely greater than at any other age. 

 It is during these years that the growth of the boy is most marked. 

 A boy between nine and thirteen years old requires just as much 

 food as a full-grown man, and between the ages of fourteen and 

 nineteen he will require more. This great increase between the 

 fourteenth and nineteenth years is not noticed in females. There is 

 a gradual absolute increase in girls up to the eleventh year. At this 

 age and henceforward the total carbon dioxide output and therefore 

 the total food requirements remain constant, being about equal to 

 that of a woman of thirty. The total energy requirements of the 

 growing animal are not, however, the only factor which we have to 

 consider. The protein of the food has not only to replace the 

 wear and tear of tissue, but also to provide material for the growth 

 of new tissues. The protein needs therefore of the growing animal 

 are relatively greater than those of the adult, and absolutely 

 greater during the period of most rapid growth, namely, in boys 

 between the ages of fourteen and nineteen. It is a popular belief 

 that in a working-class family the worker or wage-earner needs 

 a greater protein supply than the rest of the family. This is a mis- 

 take. The adult worker can obtain his energy equally well from 

 carbohydrates and fats, whereas an excess of protein is absolutely 

 necessary in the case of the children to provide the material for 

 their proper development and growth. The relation between rate 

 of growth and protein content of food is well illustrated by a com- 

 parison of the composition of the milk in different animals. In the 

 following Table (Proscher) it will be seen that the more rapidly an 



