CHAPTER LXIV 



NUTRITION AND GROWTH 



In the growth of animal tissues two factors arc concerned, one being 

 the property of the cell to grow, the growth factor; and the other, the 

 availability of suitable material to grow upon, the food factor. Concern- 

 ing the growth factor little is known ; its variability in different species 

 of animal, its irregularity despite proper adjustment of the food factors, 

 its abnormality leading to tumor formation, etc., are all well-known but 

 apparently inexplicable facts (MendeP). 



THE FOOD FACTOR OF GROWTH 



Our knowledge is constantly increasing concerning the food factor of 

 growth, and many facts of extreme practical importance have been ac- 

 cumulated in recent years. In seeking for the relationship of food to 

 growth, we must first of all consider whether this process entails a 

 greater expenditure of energy than is necessary for mere maintenance 

 in adult life. Important results bearing on this question have been se- 

 cured by observations on the basal metabolism of young children. In 

 computing the energy supply of fasting adult animals of different sizes, 

 it will be remembered that the smaller the animal, the greater is the 

 energy exchange in relationship to the body weight, although when 

 computed in relationship to body surface tolerably constant values are 

 obtained. When the calorie output per square meter is determined in 

 growing children, there is, as we have already seen, clear evidence of 

 greater energy expenditure (see page 541), particularly marked in boys 

 just before puberty. An increased energy metabolism has also been de- 

 scribed in the case of infants, but the uncontrollable muscular activity, 

 the psychic disturbances, etc., may explain the result. Even after dis- 

 counting these factors, however, it is possible that there may be a cer- 

 tain influence, depending probably on the active mass of growing proto- 

 plasmic tissue, which stimulates the energy expenditure. The question 

 is not yet finally settled. 



The Relationship of Proteins to Growth and Maintenance of Life. — 

 Since protein constitutes the fundamental chemical basis of the cell, it 

 is natural to devote attention in the first place to this food principle. 



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