THE NUTRITIVE VALUE OF FOOD 1059 



this supply must be increased considerably, thus: proteins 135 grm., 

 fat 80 grm., and carbohydrate 500 grm. This represents a total value 

 of 3348 calories. 



It will be observed that the amount of protein remains fairly con- 

 stant, while the proportion of carbohydrate and fat varies considerably. 

 Moreover, these nutritive substances may be substituted for one an- 

 other within narrow limits, but none of them should be eliminated 

 from the diet altogether, because the retention of perfect health re- 

 quires the ingestion of a certain minimum amount of each. Various 

 other factors must also be considered. For example, if there has 

 been a loss of protein material from one cause or another, it is impera- 

 tive to ingest an extra amount of protein to allow for its storage in 

 the form of tissue proteins. Quite similarly, it is desirable to increase 

 the protein metabolism during periods of training, when a perfect 

 stability of the musculature is to be attained. A limited reduction 

 in the amount of the protein is justifiable only in vigorous persons. 



Whether the prerequisite amount of protein is derived from animal 

 food or from vegetables is rather immaterial, although much quicker 

 results are obtained with the former. Both have their advantages 

 and disadvantages. While vegetables are efficient protein producers, 

 much larger quantities of them must be ingested in order to yield the 

 same degree of energy. In the end, this may not prove to be an eco- 

 nomical advantage, at least not at the present time. They possess, 

 however, certain stimulating qualities upon peristalsis and bring into 

 the body a greater variety of proteins than could possibly be introduced 

 by meat alone. It appears, therefore, that the ordinary person should 

 partake of a mixed diet rather than of one strictly vegetarian in its 

 character. 



It has been observed by Rubner that a starving animal, when fed 

 with carbohydrate, shows an increase in its heat production of from 

 30 to 40 per cent. The feeding of meat also gives rise to an increase 

 under this condition, but the increase is then almost three tunes 

 greater. It will be seen, therefore, that the proteins are actual stimu- 

 lants of metabolism and possess for this reason a specific dynamic 

 action upon the organs of metabolism. 



The Factor of Growth. In accordance with the well-established 

 fact that the intensity of the metabolism increases inversely with the 

 size of the animal, it cannot surprise us to find that children must com- 

 pensate for a much greater expenditure of energy than adults. Small 

 animals invariably lose more heat in proportion to the mass of their 

 body than large ones, although area for area of their body-surface 

 their dissipation of heat is practically the same. In order to make up 

 for this greater loss of heat, children must be more active. This is 

 true especially of boys before the age of puberty. In addition, it is 

 not at all improbable that a second factor is at work at this time in the 

 form of some stimulus derived from energized and growing protoplasm. 

 Thus, a body between the ages of nine and fourteen requires as much 



