THE ABSORPTION AND USE OF FOODS 457 



be selected. Fortunately for the well-being of the race, mankind 

 has always selected just such a diet under no other guidance than 

 liis appetite and his means, and these, to a healthy person, make 

 trustworthy guides, so long as they are accompanied by temper- 

 ance as a third. 



The importance of dietetics as a science is chiefly in connection 

 with the feeding of the sick, or providing for the maintenance of 

 large numbers of individuals, as in armies or public institutions, 

 where a slight error in selecting food, in greater amounts, or at 

 greater cost than needed, amounts in the aggregate to a very large 

 waste. 



The Nutritive Value of Albuminoids. These proteins lack some 

 of the essential constituents of cell proteins, and cannot, therefore, 

 serve as tissue-restorers. We can imagine, however, that they 

 ought to satisfy the Body's demand for protein fuel, and so be 

 substituted for the major part of the protein of the diet. Various 

 attempts have been made to substitute gelatin for proteins in this 

 way, and it seems to be highly efficacious in satisfying the Body's 

 protein-fuel demand. But curiously gelatin can be used thus for 

 only a few meals ; presently there is a revolt of the appetite against 

 it and no more can be eaten. Experiments have shown that dogs 

 will starve rather than take continuously a diet whose chief con- 

 stituent is gelatin. 



The Maintenance of Constant Weight. It is the experience of 

 most adults that during periods of unbroken health the body 

 weight remains practically unchanged day in and day out. It is 

 clear that this condition depends on the maintenance of an exact 

 balance between the intake and outgo of the Body, since if more 

 is taken in than is given out there must be a gain in weight, and 

 vice versa. It is customary to consider the question of weight 

 maintenance under three heads : water equilibrium, nitrogen equilib- 

 rium, and carbon equilibrium. 



Water Equilibrium. For a Body to be in water equilibrium 

 the amount of water lost per day must be exactly replaced by 

 the amount drank. In large measure the sudden and transient 

 changes of weight which occur are due to upsets of water equi- 

 librium. Any violent exercise in hot weather reduces the weight 

 by inducing a profuse perspiration with resulting loss of 

 water. The intense thirst which follows the exercise leads to 



