PROTEIN METABOLISM 481 



steady state is re-established with equality between intake and output. 

 In the meantime, however, the liver, kidneys, and intestinal tissues 

 have increased in both weight and protein content, and the plasma 

 proteins have also increased in quantity. Conversely there is a reduc- 

 tion in the protein of these materials to a lower level when protein 

 intake is reduced. 



Starvation amplifies this effect, and the voluntary muscles begin to 

 waste away. Heart muscle, on the other hand, is more resistant to 

 decrease and loses protein more slowly. At the beginning of fasting, 

 the glycogen reserves are used and little nitrogen is excreted. As a 

 result, the carbohydrate is said to have a sparing action for protein. 

 Limited ingestion of fats has a similar effect and delays the marked 

 losses of body protein. However, the ingestion of large quantities of 

 fat with little or no nitrogen actually accelerates the loss of nitrogen 

 from the body. Thus the storage or retention of protein depends 

 upon the levels of both protein ingestion and of carbohydrates and 

 fat in the diet. Although there are important metabolic interactions, 

 fat and carbohydrate cannot replace protein, and a balanced ration 

 is necessary. 



EXCRETION OF NITROGEN 



Excretion of various nitrogenous compounds has already been 

 referred to, including urea, uric acid, allantoin, ammonium ion, and 

 bile pigments. These compounds along with others appear in either 

 the urine or feces to account for the excretion of nitrogen. Minor 

 additional losses probably occur with perspiration and shedding of 

 cells from the skin surface. These latter factors will not be con- 

 sidered further. 



Urine 



Mammals excrete most of the nitrogen as soluble compounds in the 

 urine. The most abundant of these are listed in Table 20-3. Urea 

 is the major end product of nitrogen metabolism in all mammals, 

 although much less important in other animals. Its concentration in 

 urine is dependent upon the level of nitrogen intake and accounts 

 for the excretion of excess nitrogen after unnecessarily great ingestion 

 of protein. If protein intake is below the required level, urea excre- 

 tion is markedly reduced. 



Urea is formed by the urea cycle (page 465) and dialyzes into the 

 blood. The kidneys remove it from the circulating blood and con- 



