54 METABOLISM, NUTRITION AND DIETETICS 



drawn upon for a sudden demand, fat for a steady drain, tissue-protein 

 for a life-and-death struggle. While there may be some such difference 

 in the tenacity with which the different kinds of reserve material are 

 held back from consumption when the floating supplies are wearing 

 low, modern investigation tends to the conclusion that the interchange- 

 ability of the various groups of nutritive substances is greater than 

 had been supposed, and that in the long-run the cells in normal cir- 

 cumstances at least never work without dextrose, even after the 

 glycogen store has been practically all consumed, but secure it from 

 other sources. 



Pavy has put forward the heterodox view that the glycogen formed 

 in the liver from the sugar of the portal blood is never reconverted 

 into sugar under normal conditions, but is changed into some other 

 substance or substances, and he denies that the post-mortem formation 

 of sugar in the hepatic tissue is a true picture of what takes place 

 during life. But in spite of the brilliant manner in which he has 

 defended this thesis, both by argument and by experiment, it must be 

 said that the older doctrine of Bernard, which in the main we have 

 followed above, is attested by such a cloud of modem witnesses that it 

 seems to be firmly and finally established. 



Fate of the Sugar Glycolysis. What, now, is the fate of the 

 sugar which either passes right through the portal circulation from 

 the intestine without undergoing any change in the liver, or is 

 gradually produced from the hepatic glycogen ? When the pro- 

 portion of sugar in the blood rises above a certain low limit (about 

 1-5 or 2 parts per 1,000), some of it is excreted by the kidneys 

 (Practical Exercises, p. 716). 



A large meal of carbo-hydrates is frequently followed by a 

 temporary glycosuria, but much depends upon the form in which 

 the sugar-forming material is taken. We have seen that poly- 

 saccharides are quite incapable of absorption as such, and that they 

 must be very completely hydrolysed in the lumen of the alimentary 

 canal before their constituent sugars have any chance of reaching the 

 blood. It is therefore not to be expected that the rapid absorption 

 of such considerable quantities of sugar as would lead to its excretion 

 should easily occur when the carbo-hydrate is in this form. Miura 

 for example, after an enormous meal of rice (equivalent to 6-4 

 grammes of ash- and water-free starch per kilo of body- weight), 

 which, as he mentions, tasked even his Japanese powers of digestion 

 for such food to dispose of, found not a trace of sugar in the urine. 

 Dextrose, cane-sugar and lactose, on the other hand, when taken in 

 large amount, were in part excreted by the kidneys, as was also 

 the case with levulose and maltose in a dog (Practical Exercises, 

 p. 717).* The amount of any carbo-hydrate which can be eaten 



* Twenty-four healthy students, whose urine had previously been shown 

 to be free from sugar, ate quantities of cane-sugar varying from 250 grammes 

 to 750 grammes. The urine was collected in separate portions for twelve 

 to twenty-four hours after the meal. In only three cases was reducing sugar 

 found in the urine (by Fehling's and the phenyl-hydrazine test), and then 

 merely in traces. In eight cases cane-sugar was found, and estimated by the 

 polarimeter, and, after boiling with hydrochloric acid, by Fehling's solution. 



