108 THE PROTEIN ELEMENT IN NUTRITION 



desire for increased protein during hard labour is obscured, or 

 at least is explained away as a mere incident of the necessary 

 corresponding increase of food. 



Physiological opinion with regard to the role of protein as 

 a source of energy for muscular contraction has begun to swing 

 back to some extent from the extreme views originally held. 

 Thus Liebig believed that protein was the only food capable of 

 supplying the energy of muscular contraction. When this was 

 proved not to be the case, opinion veered round to the other 

 extreme, and carbonaceous matter was regarded as the only 

 source. At the present time it is considered most likely that 

 muscular contraction makes no special demand on one nutritive 

 constituent of the food more than another, all being made use 

 of for the supply of the necessary potential energy. When the 

 deduction is made that only comparatively small quantities of 

 nitrogen are necessary because the body is able, seemingly, to 

 burn up all the protein that is given to it within twenty-four 

 hours ; when, similarly, the fact that the urinary nitrogen is 

 not found to be greatly increased during muscular excretion is 

 taken to mean that protein is of little or no importance as a source 

 of the energy of muscular contraction, we are going beyond the 

 facts, and beyond what is warranted in the present state of our 

 knowledge. All that is certain is that, on the average, quantities 

 of nitrogen and sulphur, corresponding closely with the quantities 

 of those materials absorbed from the food, are excreted within 

 that period. The changes that may have taken place in protein 

 from the time it is broken down into the hydrolyzed products of 

 digestion until it eventually appears in the urine as nitrogen 

 are absolutely unknown. It certainly appears somewhat rash 

 to jump to the conclusion that the great proportion of the 

 protein of a standard diet is of no real service to the organism, 

 simply because the decomposition of protein keeps pace with 

 the intake, and that seemingly the nitrogen absorbed from the 

 food is rapidly eliminated. 



That protein must be taken into consideration as a source of 

 the energy of muscular work, leaving aside entirely its influence 

 on the efficiency and well-being of the body as a machine, which 

 influence cannot be measured and expressed in terms of so much 

 excreted nitrogenous waste-products, Paton's investigations 

 would make most probable. He found that the excretion of 

 nitrogen was increased both during and after the performance 



