62 PROTEIN REQUIREMENTS OF CATTLE: MITCHELL 



result. It might even be considered a matter for surprise if the great 

 acceleration in the catabolic processes of the body during muscular work 

 did not conscript some of the dietary amino acids that otherwise would be 

 used for the growth of hair or other tissues, or for the replenishment of 

 the digestive glands, or for the replacement of the nitrogenous losses 

 resulting from the minimum endogenous catabolisms. Entirely aside 

 from such admittedly hypothetical considerations as these, it would seem 

 a futile undertaking to detect the effect of muscular work upon the rate 

 of tissue catabolism that is measured normally, in the human subject, by 

 the excretion of 1.5 to 3 grams of nitrogen in the urine daily, by imposing 

 upon this an exogenous protein catabolism yielding 5 or 10 times as much 

 urinary nitrogen. 



Such considerations as these lead inevitably to the conclusion that the 

 characteristic effect of work on tissue catabolism should be investigated 

 under conditions such that the exogenous catabolism of nitrogen is 

 entirely eliminated or is reduced to an insignificant minimum. This can 

 be accomplished by the feeding of a very low-nitrogen diet, or, if possible, 

 a nitrogen-free diet. Furthermore, the experiment proper should be 

 preceded by a feeding period of sufficient length to remove the " deposit 

 protein " from the tissues, since this stored nitrogen will tend to vitiate 

 the results of the experiment in the same way as would dietary nitrogen. 

 On protein-containing but creatine-free dietaries, the excretion of 

 creatinine nitrogen is generally considered to be proportional to the total 

 endogenous nitrogen. Hence the effect of work upon the creatinine excre- 

 tion is taken to indicate the effect upon muscle endogenous catabolism. 

 Although the reasoning upon which this conclusion is based is not forti- 

 fied throughout by adequate experimental data, it may be considered 

 justifiable if not entirely convincing. 



The crucial experiments. — Of all the experiments that have been 

 reviewed so thoroughly from time to time, there appear to be only two in 

 which the diet was approximately nitrogen-free and at the same time 

 was approximately adequate in energy value, and in which the period of 

 nitrogen-free feeding was sufficiently extended so that the urinary nitro- 

 gen had approximated the endogenous level. These are the experiments 

 of ThomasC'^®) reported in 1910, and those of Kocher(2') reported in 

 1914. 



Thomas reduced his output of urinary nitrogen to approximately the 

 endogenous level by subsisting on a diet of pure sugar. Following a fore- 

 period of four days at this level, was a work period of three days during 

 which work on an ergostat was performed amounting to 120,000 kilogram- 

 meters. The experiment was concluded by an after-period of three days 



