PROTEIN REQUIREMENTS OF CATTLE: MITCHELL 61 



ever, it is equally clear that if protein is included in the experimental 

 diet, at least in quantities above the absolute minimum required to 

 replace endogenous losses of nitrogen, the significance of the urinary 

 nitrogen is difficult if not impossible to establish definitely. Under sucli 

 conditions the urine contains the products of an exogenous protein metab- 

 olism, the intensity of which for a given protein intake will vary with 

 the intensity of the prevailing anabolic processes and with the extent to 

 Avhich the protein is utilized in anabolism. Hence, even though the pro- 

 tein intake is kept constant in rest and work periods, the output of 

 urinary nitrogen might be affected if muscular activity alters either the 

 intensity of the anabolic processes or the extent to which the dietary pro- 

 tein is utilized in anabolism. It is not inconceivable that an increase in 

 muscular activity may do both, and hence a slight increase in the day's 

 urinary nitrogen in work as compared with rest periods may bear no 

 relation to the intensity of the minimum endogenous metabolism. 



On the other hand, if the energy intake is adequate, an increased 

 catabolism of muscle due to increased activity may be entirely obscured 

 if the protein intake exceeds the requirements, since dietary amino acids 

 may be diverted from catabolic to anabolic processes with no effect on the 

 output of urinary nitrogen. 



Returning now to past experiments on the effect of work on protein 

 metabolism, as reviewed for example by Cathcart, the increases in the 

 excretion of urinary nitrogen as a result of work, so frequently noted, 

 are in most cases not accompanied by a demonstration that the intake of 

 energy was adequate; in fact, in a considerable number of cases a suspi- 

 cion that the contrary was true is almost inevitable. Furthermore, in the 

 large majority of experiments, the protein intake was much above the 

 minimum requirement, and in some cases was excessive. The precise 

 interpretation of the positive results obtained with such diets seems im- 

 possible. They may have resulted from a depression of the anabolic 

 processes involving dietary protein, or from a depression of its biological 

 utilization. Although it is tnie that work tends to favor muscular hyper- 

 trophy, it seems more probable that this increase in anabolism is an after 

 effect of work rather than a contemporary effect, since the immediate 

 result of muscular contraction is undoubtedly catabolic. Even in the 

 adult, some tissues are continually growing. If this growth is inhibited 

 during muscular activity on a protein-containing dietary, the urinary 

 nitrogen will be increased by a corresponding amount. The effect of mus- 

 cular activity on the utilization of absorbed protein in metabolism is 

 entirely unknown, but it is not inconceivable tliat a depression of utiliza- 

 tion, ^nth a corresponding increase in urinary nitrogen (or sulfur) may 



