A Comparative Study of the Respiratory Quotient Following the Ingestion of Glucose and of Fructose as Affected by the Lactic Acid and Carbon Dioxide Changes in the Blood

Abstract

A study has been made of the respiratory quotient correlated with blood lactic acid and the carbon dioxide content of the blood, following the ingestion of 50 gm. glucose, 50 gm. fructose and a mixture of 25 gm. glucose and 25 gm. fructose. The respiratory exchange was determined by the open circuit method of Carpenter and Fox. It was found that an error was introduced by using discs with a small aperture (less than 1 mm. in diameter), as recommended by Simonson, for collecting aliquot samples of the expired air. The percentage composition of the gas was altered in passing through the small aperture; the gaseous mixture in the sampling bag had a higher oxygen and lower carbon dioxide percentage than that of the main gaseous stream passing through the system. When the gases passed through an aperture 1.04 mm. in diameter, the percentage composition of the mixture in the sampling bag was identical with that of the main stream. Large rubber bags, that contain approximately 10 liters of gas without exerting a back pressure, proved satisfactory for collecting gas samples through an aperture 1.04 mm. in diameter. A detailed account is given of the construction of the rubber bags used in these experiments. In five experiments in which 50 gm. glucose was ingested there was generally no increase in the lactic acid nor decrease in the carbon dioxide content of the blood. It was concluded that the respiratory quotients obtained within 30 minutes after the ingestion of glucose were true metabolic quotients. The ingestion of the same quantity of fructose led in each instance, within 15 minutes and later, to an increase in blood lactic acid and an attendant decrease in the carbon dioxide content of the blood. The same events followed the ingestion of a mixture of 25 gm. glucose and 25 gm. fructose. The average lactic acid and carbon dioxide changes observed with this mixture, were approximately of the same magnitude as those observed with 50 gm. fructose. From these observations, it is evident that the respiratory quotient obtained after the ingestion of fructose is not a true metabolic quotient. When allowance was made for the blowing off of nonmetabolic carbon dioxide, the respiratory quotients obtained over a period of 30 to 45 minutes after the ingestion of fructose, were considerably higher than the quotients obtained over the corresponding time after the ingestion of glucose. The difference indicates either a combustion of relatively more carbohydrate after the ingestion of fructose, or a transformation of this sugar into fat. When the respiratory quotients obtained with fructose were above unity and these high quotients were corrected for the non-metabolic carbon dioxide, as calculated from the increase in the lactic acid and from the decrease in the carbon dioxide content of the blood, they still remained above unity. It is therefore concluded that a portion of the ingested fructose was converted into fat.