104 ELEMENTARY BIOLOGY 



the taste. Since the high-protein foods are mostly of animal 

 origin, and the low-protein foods are mostly of vegetable origin, 

 a balanced ration selected to meet all three requirements men- 

 tioned above (bulk, protein ratio, and taste) is likely to contain 

 materials of both kinds. At any rate, it is only by means of a 

 mixed diet that we are able to maintain for a long time a satis- 

 factory ration. Milk for children less than a year old would 

 seem to be the only exception to this statement. 



Experiments with mice and guinea pigs, as well as with human 

 beings and other animals, have shown that the various proteins in 

 the materials used as food do not all behave alike in relation to 

 growth, or in relation to maintaining body weight. The chemical 

 analysis of proteins that behave in these different ways shows that 

 certain groups of elements, called amino adds, which are contained in 

 some proteins, are absolutely necessary for growth, whereas other 

 amino acids are suitable for maintaining weight, although the\ 

 cannot support growth. 



The importance of having the diet balanced appears among 

 people who are either so ignorant as to purchase food entirely 

 on the basis of the appetite or the temptations of the market, 

 or so poor as to be unable to buy any but the cheapest articles 

 to be obtained. Hie indulgence of the appetite may lead to 

 malnutrition through an excess of sweets, and to digestive dis- 

 turbances through an excess of meats (proteins). The resort to 

 the cheapest foods may lead to malnutrition through an excess 

 of starches, since, generallv speaking, the starchy foods are the 

 cheapest, weight for weight. There are other matters, besides 

 the nutritive ratio, that influence the physical condition of the 

 body ; but this is something that cannot be safely disregarded. 



