The Effect of Glucose and of Fructose on the Human Respiratory Quotient and Alveolar Air

Abstract

The respiratory exchange and composition of the alveolar air were determined with two human subjects on 3 and 4 days for four post-absorptive 15-minute periods and for ten 15-minute periods after the ingestion of 25 grams of glucose and of fructose. There was no change in the alveolar carbon dioxide with the trained subject accompanying the rises in the respiratory quotient after the ingestion of the sugars, but with an untrained subject there was a marked fall in the alveolar carbon dioxide during the periods of maximum rise in the respiratory quotient. With the trained subject there was not a definite relationship between the alveolar carbon dioxide and the alveolar respiratory quotient in the experiments with fructose, but in the experiments with glucose and in the two groups of experiments with the untrained subject there was a tendency to a negative correlation between the alveolar carbon dioxide and the alveolar respiratory quotient. There was a marked negative correlation between the alveolar oxygen deficit and the alveolar respiratory quotient in both groups of experiments with both subjects. The alveolar respiratory quotients tended to run parallel with the respiratory quotients of the expired air. There was a maximum rise in the respiratory quotient of 0.065 in the experiments with glucose with one subject and 0.10 with the other subject. When the respiratory quotient after the ingestion of glucose with the untrained subject was corrected for the rise in respiratory quotient corresponding to the falls in the alveolar carbon dioxide equivalent to those which took place after the ingestion of glucose, the net increase in the respiratory quotient was then the same as with the trained subject. The average maximum rise in the respiratory quotient of the expired air with the trained subject in the experiments with fructose was 0.11 and was accompanied by nearly the same rise in the alveolar respiratory quotient with no significant change in the alveolar carbon dioxide. The average maximum rise in the respiratory quotient of the expired air with the untrained subject was 0.16, accompanied by a rise of 0.17 in the alveolar respiratory quotient and a decrease in the alveolar carbon dioxide of 0.4 per cent. When the respiratory quotient of the expired air is corrected for the apparent rise due to a fall of alveolar carbon dioxide found in previous post-absorptive experiments with this subject, the increase in the respiratory quotient of the expired air becomes 0.12, thus practically the same as with the trained subject. Therefore a comparison of the course of the alveolar air and the respiratory quotient in the post-absorptive condition with the changes taking place after the ingestion of sugars gives a basis for the calculation of the true effect of the ingested sugars on the respiratory exchange. If the constancy of the alveolar air is to be taken as an indication of true metabolic respiratory quotients, as has been frequently suggested in the literature, then the net rise in respiratory quotients in these experiments as the result of ingestion of fructose must be regarded as a result of the metabolism of fructose without the formation of organic acids in its transformation in sufficient quantities to affect the carbon dioxide tension of the alveolar air.