478 ANIMAL BIOCHEMISTRY 



Normal adults on normal diets thus should always be in nitrogen 

 balance. If physical activity increases the demand for nitrogen needed 

 in repairing muscle tissue, appetite increases too. When an individual 

 increases his food intake without a change in actual need, the excess 

 protein is stored to a limited extent at first. Thereafter, the excess 

 is converted to fat for storage and the nitrogen eliminated. Con- 

 versely, a limited decrease in protein consumption first eliminates the 

 conversion to fat and reduces the limited storage, so that intake and 

 output again equate but at a somewhat lower level. 



However, when protein intake is sharply ciatailed, nitrogen excre- 

 tion cannot decrease correspondingly and continues to exceed the 

 supply. Such a state, called negative nitrogen balance, arises during 

 starvation, dieting to reduce total body weight, wasting illnesses like 

 fevers, and improper diets deficient in materials essential for protein 

 utilization. Obviously a prolonged negative nitrogen balance will be 

 harmful and may lead to kwashiorkor, for example. 



On the other hand, young growing animals ingest more nitrogen 

 than they excrete, using the difference for cell multiplications and 

 tissue increases. This situation is called positive nitrogen balance. 

 Normal adults do not show this behavior. However, positive nitrogen 

 balance does occur during pregnancy, recovery from loss of blood or 

 wasting diseases, wound healing, or very briefly when the level of 

 protein intake rises appreciably. After parturition or the repair of 

 tissue is completed, the individual returns to nitrogen balance until 

 the next departure from normality causes a shift in either the positive 

 or the negative direction. 



Essential Amino Acids 



For experimental purposes animals have often been fed diets con- 

 taining all known dietary factors but with only a single protein. When 

 the level of the protein in such a diet is varied, that level just sufficient 

 to allow nitrogen balance is taken as the quantity required for well- 

 being. When different proteins are studied in this way, different 

 minimum amounts are required. This variation is now attributed to 

 differences in protein structure, and most of it to variations in the 

 amino acid composition of the proteins. 



Studies on the nature of this effect employ mixtures of the amino 

 acids found in proteins added to the otherwise complete rations 

 instead of protein. By omitting the amino acids one at a time, it is 

 possible to discover those that must be supplied as distinguished from 

 those that can be synthesized by the test animal. Furthermore, 



