PROTEIN REQUIREMENTS OF CATTLE: MITCHELL 63 



on the same diet. The daily excretions of urinary nitrogen were as fol- 

 lows: fore-period, 3.05, 2.73, 3.32, and 2.85 gms. ; work-period, 2.47, 2.90, 

 and 2.97 gms.; and after-period, 2.71, 2.22, and 2.31 gms. Judged by a 

 comparison of the average excretions only the work had no effect on the ex- 

 cretion of urinary nitrogen, the averages being, respectively, 2.96, 2.78, 

 and 2.41 gms. If it is considered that in the latter period only was the 

 endogenous level of tissue catabolism reached, Thomas concludes that the 

 work might have effected a slight increase in muscle catabolism. However, 

 if the endogenous catabolism were not attained until the last few days of 

 the experiment, the increased excretion of nitrogen in the work period 

 above this level would appear to be due more probably to the catabolism of 

 " deposit protein " than to an effect of work. This experiment of Thomas, 

 therefore, does not support the conclusion that muscular work increases 

 the endogenous muscular catabolism. It is to be noted that definite 

 proof of the adequacy of the diet in energy value during the work period, 

 by a comparison of intake with outgo of energy, is not required when the 

 urinary nitrogen shows no increase in this period. 



Kocher's experiment involved two subjects and a study of the nitrog- 

 enous constituents in the urine. The diet used contained a minimal 

 amount of nitrogen (1.01 gms. per day) derived entirely from cream and 

 contained over 5000 cals. daily. In each case there was but one work 

 day, during which the subject walked 60 kilometers (about 37.8 miles) 

 in 10 hours. AVith both subjects the total urinary nitrogen and the creat- 

 inine nitrogen increased on the day of work, and the increased excretion 

 continued for the next day or two. The effect is much more noticeable in 

 the first subject than in the second, a fact possibly correlated with the 

 much lower creatinine coefficient of the first subject. Unfortunately there 

 is not sufficient assurance that the calorie intake of the work days was 

 adequate. On the basis of average values for the energy requirements at 

 rest and during horizontal walking, Kocher estimates that the energy 

 intake of Subject E. A. K. was about 4.0 per cent in excess of the 

 requirements of the day of work ; with Subject J. G. F. the estimated 

 excess was 4.6 per cent. However, the estimates of energy requirements 

 might conceivably be in error by much more than this. Hence, the ex- 

 periments do not demonstrate that an increase in endogenous catabolism 

 is an inevitable consequence of muscular work ; in other words, that there 

 is an inevitable wear on the muscle machine. 



In further experiments on the same subjects, using a high-protein 

 high-fat diet, the urinary nitrogen showed no increase as a result of 

 work when the calorie intake was high, but a distinct increase when the 

 calorie intake was deliberately lowered so as to be obviously insufficient. 



