METABOLISM OF MATTER AXD ENERGY. 1005 



but little is known regarding the kind and amount necessary. Protein, 

 fat, and carbohydrates all furnish energy, but protein alone furnishes 

 nitrogen. The above statement may be further simplified by saying 

 that a definite amount of nitrogen and energy is essential- 



(5) A more abundant ration is required for muscular work, for fatten- 

 ing, and for milk production than for maintenance; that is, gains in 

 weight, milk, and force for muscular power are produced at the expense 

 of nutrients supplied in excess of the amount required for maintenance. 



(f>) Food supplied in excess of all needs is stored, in part at least, as 

 reserve material. The principal reserve materials, fat and glycogen, 

 contain no nitrogen, but it is reasonably certain that under certain 

 circumstances a small amount of some reserve material containing 

 nitrogen is also stored up. 



(7) If no food is supplied or a- diet containing no nitrogen is con- 

 sumed, nitrogen (derived from body tissue) is still excreted in the 

 urine; that is, the animal is living upon its own substance wholly or in 

 part. Under these circumstances the urine and feces contain more 

 nitrogen than the food; that is, tin 1 body is losing nitrogen. When the 

 food contains more nitrogen than is needed there is usually, for a time 

 at least, a small gain of nitrogen in the body, i. e., the urine contains 

 less than the food. However, it does not follow that an excess of 

 nitrogen consumed over the amount essential is stored up in the body. 

 As was mentioned above, the body under ordinary circumstances 

 adjusts itself to new conditions and uses up the nitrogen supplied to 

 it, although it is not altogether certain just how this is accomplished. 

 In other words, the body comes into nitrogen equilibrium ; that is, the 

 outgo of nitrogen approximates the income. If all disturbing factors 

 could be controlled, it would probably exactly equal the income, but 

 this can not be said with certainty. 



If it were not for the fact that the body comes into nitrogen equi- 

 librium with rations containing different amounts of nitrogen, it is 

 evident that it would continue to form muscular tissue, i. e., cell and 

 cell contents, indefinitely on a ration rich in nitrogen: since muscular 

 tissue (including organs and blood) is the portion of the body char- 

 acterized by nitrogen. Under some conditions of feeding there are 

 undoubtedly small gains in nitrogen, i. e., muscular tissue. 1 But it is 

 a matter of common observation that overfeeding does not produce 

 more lean, though it does produce more fat. 



(8) Fat, which is stored as reserve material, is formed from fat and 

 carbohydrates supplied in the food, and doubtless from protein also. 



(9) As furnishers of energy the different nutrients may replace each 

 other in approximately the following ratio: Protein : fat : carbohy- 



1 How much of the nitrogen thus stored forms a part of the cell tissue, how much 

 belongs to cell contents, and how much is in circulation in fluids in the body is a 

 question in regard to which opinions differ, and the data for judging this matter 

 are at present inadequate. 



