METABOLISM 179 



portant and far-reaching a nature. It is possible that on mini- 

 mum protein intake, reserve stores of important but seldom 

 needed materials might be seriously depleted, thus reducing the 

 body's power of resisting disease or meeting emergency demands. 

 In this connection, the work of Haecker is most interesting. A 

 herd of cattle was kept for about three years on low protein 

 diet. The first two years all went well. In the third year the 

 animals showed lessened resistance to the inroads of disease, and 

 finally became so ill that the experiment was discontinued. 



In summary, perhaps the older standards of about 120 

 grams protein are unnecessarily high. A compromise on per- 

 haps 90 grams of protein for the average adult has been sug- 

 gested as the most satisfactory solution of the problem. 



The fact that nitrogen equilibrium can be maintained on so 

 low a level as 2-3 grams N a day makes it seem possible that the 

 breaking down of the protein tissues takes place to a much 

 smaller extent than was supposed. Another and a more prob- 

 able explanation of this fact is that much of the material pro- 

 duced in the destruction of protein tissues is used again in the 

 body to rebuild the destroyed tissues. 



The maximum amount of protein on which nitrogen balance 

 may be maintained is of little practical interest. It will be 

 limited by the ability of the organism to absorb protein products 

 from the digestive tract. Excess protein is simply destroyed in 

 the organism. In addition to the economic disadvantage of so 

 expensive a diet, unnecessary strain is put on the excretory or- 

 gans in disposing of the excess of urea and other end products 

 formed. It is an interesting fact that increased muscular ac- 

 tivity does not increase nitrogen excretion if the animal is well 

 nourished. The animal burns carbohydrates and fats, and de- 

 stroys no more protein. 



Thus far we have spoken only of protein need. In so doing, 

 we really are considering ammo acid need, for the proteins are 

 broken down to amino acids in digestion. Since proteins contain 

 widely varying proportions of the different amino acids, and a 

 given protein often lacks one or more of these compounds, it is 



