THE PROTEIN REQUIREMENTS OF MANKIND 109 



of work, and the metabolism of protein indicated by the increased 

 nitrogen excretion amounted to 35 per cent, of the work done. 

 The conclusion arrived at is that " when the body contains no 

 stored protein, muscular exertion increases the output of nitrogen 

 15 per cent. ; but if the body is in good condition " that is, 

 containing a store of circulating proteins " the output of 

 nitrogen is larger, because such proteins are consumed as a source 

 of heat and energy ; but the muscle fibres are not broken down in 

 a greater proportion by the work they perform."* 



Even if the excretion of nitrogen were entirely unaffected by 

 work, this would be no proof that none of the energy of muscular 

 contraction comes from proteins. The excretion of nitrogen 

 would be unaltered when proteins are oxidized within the body, 

 whether in the one case their energy appeared in the form of 

 heat, or in the other case it became diverted to muscular con- 

 traction. 



In connection with this conception it is probable that in the 

 work of the body generally, and in the work of muscle in par- 

 ticular, protein may be made use of according to the richness 

 and liberality of the supply ; in fact, it is known beyond doubt 

 that the organism is prodigal of its protein when there is an 

 abundance offered in the food, and most economical when the 

 opposite conditions obtain. It can easily be imagined that with 

 a free supply of protein the interchanges taking place in the 

 nitrogenous tissues are ranged at a high level, the old building- 

 stones are rapidly got rid of, and their place occupied by others 

 manufactured from the amino-acids of digestion. This would 

 not mean any increase in the excretion of nitrogen beyond that 

 which would be the case on the theory that a very large propor- 

 tion of the protein is short-circuited through the liver, changed 

 into urea, and rapidly eliminated. 



In the opposite condition, when the supply of protein is 

 eventually cut off, or greatly diminished, the tissue cells, instead 

 of eliminating their old or damaged building-stones, make use 

 of them in the best manner possible to repair the waste essential 

 to body work and growth, and even cut down the protein inter- 

 changes to the lowest possible level. This would mean that, 

 although during muscular exertion there may be no great increase 

 in the excretion of nitrogen, the tissue cells, on the other hand, 



* Tibbies, " The Protein Requirement," British Medical Journal, vol. ii., 

 1911. 



