210 NUTRITION 



have been given in liberal amount, measured by its nitrogen 

 content. In other words, the character of the protein is of 

 more importance than the quantity. 



In the diet of civilised communities this question does 

 not often arise owing to the fact that man has obeyed his 

 instinct in taking a large and varied protein diet, thus 

 ensuring that every amino-acid will be present in adequate 

 amount. But when the protein intake is reduced, as in 

 Chittenden's experiments, it becomes a question whether 

 the border-hne is not reached so far as any individual 

 amino-acid is concerned. 



Of recent years many experiments have been performed 

 to demonstrate the need for individual amino-acids. The 

 pioneer work was performed by Hopkins in 1906. Hopkins 

 fed rats on a diet of protein, fat and carbohydrate, in 

 which the protein took the form of zein — a protein deficient 

 in tryptophane, lysine and glycine. Though the diet was 

 abundant as regards its calorie value, the animals lost flesh 

 and died within one to four weeks. On adding tryptophane 

 to the diet, they lived some time longer and for a time 

 maintained their weight. Later experiments have shown 

 that on adding lysine as well as tryptophane growth and 

 health are restored. Lysine and tryptophane, therefore, 

 are needed by the hving tissues. 



Maintenance and Growth 



Nutrition is adequate in the adult when it maintains 

 the efficiency of the body, and in the young when in 

 addition to this it provides for the normal rate of growth. 

 What is the normal rate of growth? Growth depends 

 upon two factors — the growth factor and the food factor. 

 The growth factor is the inherent tendency to grow, which 

 is subject to individual variations, depending upon the 

 laws of heredity. It sets the upper limit to growth which 

 no amount of feeding can overstep. The part which the 

 food factor plays lies in providing the material upon which 

 the growth factor can work. The normal rate of growth 



