900 NUTRITION AND HEAT REGULATION. 



in a diminution of diseases of the kidney. The newer conceptions 

 in regard to the digestion and nutritive history of the protein 

 foods certainly seem to favor the adoption of a low protein diet. 

 If protein is eaten in excess of the real assimilation needs of the 

 tissues, all the excess, so far as we can see, might just as well be 

 substituted by carbohydrate or by carbohydrate and fat. The 

 excess nitrogen thus eaten appears to be so much useless ballast 

 which the body very promptly gets rid of. The uncertain point, 

 however, is what constitutes the assimilation need of the" tissues. 

 The experiments given above would place this need very low, 

 according to the lowest estimate, at about 5 per cent, of the total 

 energy value of the food. That is to say, if the daily diet contains 

 heat energy equivalent to 2400 calories, only 5 per cent, of this, 

 120 calories, needs to be in the form of protein, an estimate which 

 would bring the protein to about 30 gms. daily. Many objections 

 have been urged against the attempt to apply this apparently 

 logical conclusion to public dietaries. We are reminded that our 

 knowledge of the details of the metabolism of protein is very in- 

 complete, and it is not certain yet that in the long run a low protein 

 diet would be entirely without injury. Emphasis has been laid 

 also upon the importance of providing what Meltzer has named a 

 "f actor of safety," that is to say, a certain margin beyond a bare 

 sufficiency which will be a reserve against unusual demands, but 

 this objection simply serves to restate the question. If a man is 

 accustomed to eat 100 gms. of protein daily and science demon- 

 strates that he can maintain a nitrogen equilibrium on 30 gms., does 

 a reasonable factor of safety require the use of the additional 70 

 gms., or would perhaps a total of 50 or 60 gms. per day meet every 

 requirement? It seems obvious that we must find out the truth 

 in this matter by observation and experiment. There is one con- 

 sideration that should induce us to be cautious in making a radical 

 change in the dieting of collections of individuals, such as armies, 

 institutions, etc., and that is, that our instinctive appetite seems to 

 lead us to seek the higher protein diet. We know very little, indeed, 

 about the mechanism of these appetites, but as a matter of fact, 

 they and not the results of science control the amount and kind of 

 food that we eat. Science is rapidly gaining information that 

 enables us to guide or control the appetite for food in a conscious 

 and reasonable way, but at present our knowledge is not sufficient 

 to warrant disregarding the bodily sensations when, as in the case 

 in question, they seem to cause a similar reaction in normal men 

 under varying conditions. Studies of dietaries seem to show that 

 mankind, left to the guidance of the natural appetites, has always, 

 when possible, adopted the high protein level of 90 to 100 gms. per 

 day. Indeed, the uniformity with which this level has been un- 



