1 68 SCIENCE PROGRESS 



body seems to be determined by the amount of proteid eaten 

 rather than by the presumptive needs of the tissues for a supply 

 of energy. An increased elimination rapidly follows consump- 

 tion, and well-nigh the whole of the nitrogen ingested in the 

 form of proteid appears in the urine within twenty-four hours. 



The excretion is equally complete if the amount of proteid 

 taken be increased greatly, and it goes on just the same if the 

 consumer remains completely at rest on a diet amply sufficient 

 for hard work. The adult human body does not store extra 

 nitrogen (or stores it only under very exceptional circumstances), 

 and so, clearly, does not retain any excess of proteid. But, if 

 so, it would seem that proteid as a source of energy suffers a 

 great disadvantage when compared with fats or carbohydrates. 

 These can be stored and used at convenience ; the utilisation 

 of proteid, on the other hand, is apparently compelled by its 

 mere arrival, and cannot await the needs and conveniences ol 

 the body. 



It is the great merit of a theory recently advanced by Folin 

 that it enables us to escape from this paradoxical view. In the 

 light of Folin's theory the rate of nitrogen excretion through the 

 kidney shows itself, not as a measure of the rate in which energy 

 is being extracted from proteids, but as an indicator of the 

 activity of a process whereby the nitrogen is immediately 

 removed from the amino-acids formed in the bowel, while their 

 non-nitrogenous remainders are used subsequently and at 

 convenience. Such removal of nitrogen occurs without practical 

 loss of energy. 



It was shown, two years ago, by Lang in Hofmeister's 

 laboratory, that the tissues, generally, have the power of 

 removing nitrogen from amino-acids in the form of ammonia ; 

 and it has since become clear that the direct removal of the 

 amino group (NH 2 ) as ammonia from various substances is 

 a characteristic chemical event in the body. It may occur by a 

 simple process of reduction, or by one of hydrolysis, as when 

 alanine (amino-propionic acid) is converted into lactic acid, 



CH 3 CHNH 2 COOH + H 2 = CH 3 CHOHCOOH + NH 3 . 



Now, during proteid digestion ammonia is undoubtedly in- 

 creased in the portal blood [Nencki and Zaleski ; Folin 2 ], and 

 being carried to the liver becomes the precursor of urea, 

 which is rapidly eliminated. Folin's own very elaborate studies 



