GO PROTEIN REQUIREMENTS OF CATTLE: MITCHELL 



Considering the average urinary excretions of rest and work periods, 

 Shaffer and Lusk both conclude that work has had no effect on the nitrog- 

 enous metabolism. Cathcart, however, notes a marked increase in uri- 

 nary nitrogen on the fourth Avork day, and, without reference to the rest 

 periods at all, implies that this is a result of the work performed. Cath- 

 cart's recent experiments C*), although very carefully controlled and 

 undivided in their obvious interpretation, seem no more convincing than 

 the much earlier experiments of Wait^^) at the University of Ten- 

 nessee, which yielded entirely negative results. These experiments of 

 Wait are not considered in Cathcart's review. 



A definition of the problem and its bearing on experiniental metliods. — 

 The maze of conflicting experimental results relative to the effect of work 

 on protein metabolism suggests that a precise definition of the real prob- 

 lem, followed by a consideration of the proper experimental conditions to 

 impose, might be of value in deciding which of the recorded experiments 

 are capable of the most exact interpretation. Since there is now no ques- 

 tion of the availability of protein as a source of energy for muscular work, 

 the real problem becomes that of determining whether or not there is an 

 inevitable disintegration of muscle protein (or of other nitrogenous com- 

 pounds) as a result of muscular activity. 



This definition of the problem imposes certain necessary conditions 

 upon any experimental method of attack. In the first place, there should 

 be an adequate intake of energy during the working periods, otherwise an 

 increase in urinary nitrogen may mean a destruction of tissue protein 

 merely to serve the unnecessary role of a source of energy. If no evidence 

 is obtained of an increase in tissue destruction as the result of work, 

 this in itself would seem to be satisfactory evidence that the energy intake 

 was adequate. On the contrary, if the experimental results indicate 

 an increased destruction of tissue, the investigator himself must assume 

 the burden of the proof that the energy intake was adequate, and hence 

 that the increased tissue catabolism was an inevitable consequence of 

 muscular activity. Thus, the significance of negative results in the 

 investigation of this problem would seem to be much more easily estab- 

 lished than the significance of positive results. 



The only known method of investigating the extent of the destruction 

 of tissue protein under any experimental conditions is to determine the 

 nitrogen content of the urine on a nitrogen-free diet of adequate energy 

 value, and even under these conditions the contribution of the non- 

 protein nitrogenous constituents to the urinary nitrogen cannot be de- 

 termined. It would seem to be impossible, therefore, to rule out the role 

 of the latter constituents in the phenomena under investigation. How- 



