THE GENERAL NATURE OF DIET 153 



his food, at least five-sixths are expended in warming the body to its 

 constant temperature. A large part of this is expended from the skin. 

 Any variation in the radiation, conduction, and evaporation from the 

 skin, therefore, would markedly affect the amount of food required. In 

 bodies of the same general shape, but of different size, the surface area 

 is much larger in proportion to the mass in a smaller body than in a larger 

 one. It is by the mass that the heat is produced, and by the surface 

 area that it is largely lost. It is on this principle partly that a child 

 requires proportionally much more food than does an adult. A male 

 baby of one year requires nearly twice as much food proportionally to his 

 weight as does the man twenty years old doing moderate muscular work. 

 Another factor in this difference is the greater activity of the metabolism 

 in children. Moreover, in early life the body is growing larger, and more 

 food is required on this account. 



In general terms, then, the man who works with his muscles outdoors 

 in winter requires the largest amount of food, while the man who works 

 with his brains indoors requires proportionally the smallest amount of 

 food. As we have noted, however, the latter individual consumes more 

 nearly the food that the former consumes than what we might expect 

 theoretically. It remains to be seen by further research whether the 

 indoor brain-worker in particular actually eats on the average more than 

 he needs. 



Qualitative Adaptation in Certain Physiological Conditions is Valuable. 

 The last demand in an ideal diet which was suggested was that the 

 food during certain physiological periods and conditions should be 

 adapted qualitatively to their respective needs. 



Stating the matter in this way, the implication is conspicuous that, 

 save in these circumstances, diet should not be adapted qualitatively. 

 Such undoubtedly is the case. From considerations already discussed, 

 it is obvious that it is part of the power peculiar to protoplasm to select 

 for itself out of the circulation's general store of pabulum that which it 

 needs for its own, perhaps unique, purposes, and to reject what it has no 

 use for. This selective capacity of bioplasm is everywhere conspicuous, 

 and forms one of the most marked characteristics of the living substance. 

 A less specialized selection of nutritious material is made by the absorp- 

 tive mechanism of the gut, and this determines what shall enter the cir- 

 culation. A still more general choice is made by the digestive enzymes, 

 and they determine, probably ionically, what things shall and what shall 

 not gain access to the absorptive selecting cells. The most general 

 selecting agent of all is the will of the individual at large, and his choice 

 of food is guided by convenience and by his intelligence plus his appetite, 

 and bids him eat what is good for him by general consent. He learns, 

 unconsciously perhaps, that in order to live he must eat proteid, fat, 

 carbohydrate, salts, and water, each and all, and that the proportions of 

 these and their total amount must both be approximately right. Further 

 than this, the individual makes no selection other than that which his 

 appetite's caprice or habit may command. He leaves it to the delicately 



