THE CAEBOHYDRATES AND THEIR METABOLISM 253 



at no time do we find an accumulation above those levels. If, however, 

 we ingest a large amount of carbohydrates in the form of glucose which 

 requires no digestion at all, and which is absorbed with great rapidity, 

 we find that glucose enters the blood stream at such a rapid pace that 

 the three outlets utilization, glycogen formation, fat formation are not 

 sufficient to remove it all. Its concentration in the blood stream rises 

 and we develop what is known as a condition of liyperylucemia. 



Another process may be brought into play at this stage, namely that 

 of ylucosuria. 



It is a well-known fact that the kidneys exercise a selective action on the 

 substances that circulate through it in the blood stream. At the present 

 state of our physicochemical knowledge it is difficult to say what the 

 mechanism of kidney secretion is. But we do know that for a number 

 of crystalloids the rate and amount of their excretion bears a definite re- 

 lationship to their concentration in the blood. (Ainhard and Weil, 1914; 

 McClean, F. C., 1015.) 



The behavior of glucose in the blood is like that of a pure crystalloid 

 (Michaelis and Rona, 1908), and one would expect the kidneys to per- 

 mit its free secretion in the urine. This, however, is not the case. With 

 the ordinary reduction tests (Fehling's solution, Benedict's solution, etc.) 

 we cannot detect the presence of glucose in the urine of normal indi- 

 viduals 7 if the blood sugar concentration fluctuates within the normal 

 limits. When, however, the concentration of glucose in the blood rises, 

 there comes a point at which the kidneys begin to excrete it in easily de- 

 tectible quantities. 



The height of blood sugar concentration at which the kidneys begin 

 to secrete sugar differs with different individuals and is known as the 

 kidney threshold for sugar. With a very few it lies as low as 0.08 per 

 cent, which means that these people excrete glucose in detectible quanti- 

 ties all the time, and they suffer from a condition that is recognized as 

 renal glucosurla. Others will not excrete it even when the concentration is 

 as high as 0.26 per cent, as in cases of chronic nephritis. These two ex- 

 tremes are comparatively rare. The great majority of normal individuals, 

 however, excrete glucose in the urine in detectible quantities when the 

 glucose concentration of the blood rises above 0.15 to 0.16 per cent. There 

 is at present no explanation for this individual variation, except for the 

 statement that there must exist a difference in sensitiveness for glucose in 



'Stanley R. Benedict has recently reported (1918) that the urine of a normal dog 

 and of two. normal, men can be shown to contain substances which are fermentible by 

 yeast and which reduce picric acid. He assumes that it is glucose. The dog weighing 

 18 kilos excreted in the neighborhood of 390 mgs. per 24 hours when kept on an ordi- 

 nary carbohydrate diet; 281 rngs. when kej)t on a low carbohydrate diet; 194 ings, when 

 fasting. His human subject, E. O., weighing 86 kilos, excreted 996 mgs. per 24 hours 

 when on an ordinary carbohydrate diet; 777 mgs. when on a low carbohydrate diet; 

 1479 mgs. when on a carbohydrate-rich diet. The second subject, weighing 57 kilos, 

 excreted 640 mgs. when on an ordinary diet; 543 mgs. when on a low carbohydrate diet; 

 847, '1156 and 1528 mgs. on eacli of three days of carbohydrate diet. 



