124 BIO-CHEMICAL JOURNAL 



In quite recent years however, another, and almost diametrically 

 opposed hypothesis, has been regarded with much favour, and has 

 found an admirable exponent, amongst others, in the person of Speck. 1 

 According to this hypothesis, the nitrogen is rapidly eliminated from 

 the products of digestion of the albumens, and excreted in the form of 

 urea, whilst the carbohydrate part of the molecule forms, on oxidation, 

 the main source of the energy supply of the organism. The tissues 

 themselves undergo degradation only under exceptional circumstances, 

 as, for example, when the food supply is insufficient for the energy 

 needs, or when the conditions of metabolism are abnormal, as is the 

 case during fever, or when the oxygen supply is insufficient, or after 

 phosphorus poisoning. These abnormal conditions will be discussed 

 again later. According to Speck, therefore, we must distinguish 

 between the metabolism of energy and the metabolism of tissue, and 

 as the main source of energy is derived from the oxidation of the 

 non-nitrogenous part of the molecule, the albumens should play a 

 comparatively subordinate role in the general economy of nutrition. 

 Some support is given to this conception by the recent researches of 

 Siven* and of Chittenden, 3 ' both of whom find that equilibrium can be 

 maintained on diets that contain very much less nitrogen than was 

 supposed to be necessary by the earlier investigators. 



An important new view of the matter has been advanced recently 

 by Folin, 4 which does not differ very materially from that advocated 

 by Speck. Folin has shown that the change from a nitrogen-rich to 

 a nitrogen-poor diet is followed by marked changes in the composition 

 of the urine. Not only does the urea output diminish in actual 

 quantity ; its relative quantity in comparison with the other nitrogenous 

 constituents of the urine also diminishes. On the other hand, the 

 creatinine output remains always constant, whether the diet be nitrogen- 

 rich or nitrogen-poor. The neutral sulphur of the urine maintains a 

 similar constancy. From these facts Folin concludes that metabolism 

 can be considered under two heads, viz. : as endogenous metabolism, 

 which is due to a constant tissue waste (and is analogous, therefore, 

 to the tissue metabolism of Speck), which is responsible for the con- 

 stituents of the urine (creatinine and neutral sulphur) which do not 



