178 NUTRITIONAL PHYSIOLOGY 



burning of protein. Urea itself has a certain capacity for 

 oxidation and a low but distinct fuel value, something 

 like 2.5 Cal. per gram. The residual fuel value in urea is a 

 sign that some portion of the energy latent in protein is 

 constantly lost to the animal economy. Bacteria may 

 profit by it, but it seems not to be available for the higher 

 living forms. There are certain products other than urea 

 which remain after the decomposition of protein molecules 

 and which likewise represent unused energy. Some of 

 these minor products accompany the urea in the urine, 

 while others are mingled with the feces. To make ac- 

 curate allowance for the energy lost with these incompletely 

 oxidized compounds is a difficult matter. The estimate 

 arrived at credits to a gram of protein 4 Cal. or a little 

 more. It is a coincidence with very convenient results 

 that this is almost exactly the same as the figure for car- 

 bohydrate. 



The Total Daily Metabolism. The widest limits be- 

 tween which the metabolism of an adult may vary may be 

 set down as 1000 and 10,000 Cal. per day. The lowest 

 level will be approached when there is complete rest and 

 protection from cold during the twenty-four hours. The 

 maximum will be reached, if ever, when a large, powerful 

 man performs the heaviest muscular work while under ob- 

 servation. The food taken on a single day influences 

 the result less than would be anticipated. (As already 

 pointed out, the appetite follows the metabolism rather 

 than precedes it.) High protein feeding has an effect 

 which will be discussed later. 



Of course, the average for an individual will generally 

 fluctuate much less than is suggested above. It will 

 rarely fall below 1500 or rise above 3000 under the con- 

 ditions of city life. Until recently it would have appeared 

 reasonable to fix upon 2400 as a mean. This value has 

 lately come to be regarded as higher than the actual energy 

 output of most people. Perhaps 2000 may be adopted as 

 an ordinary amount. For sixteen hours of waking we 

 may allow 100 Cal. per hour, and for eight hours of sleep 



