The Effect of Various Amounts of Caffeine on the Gaseous Exchange and the Respiratory Quotient in Man

and Conclusions The effect of caffeine on the respiratory exchange has been studied on two subjects with dosages of 1, 3 and 6 mg. caffeine alkaloid per kilogram body weight. One milligram caffeine per kilogram produced no effect on the oxygen consumption, carbon dioxide elimination or respiratory rate on either subject, nor in the respiratory quotient or total ventilation in one subject. In the other subject it produced a rise in the respiratory quotient and a slight increase in pulmonary ventilation. The two larger doses were followed in the first and second 15-minute periods by a rise in the respiratory quotient, oxygen consumption and carbon dioxide elimination in both subjects. The rise in the quotient was followed by an abrupt descent which carried the quotient below the base line and below the level of the corresponding periods in the control experiments. The effects were more pronounced with the 6 mg. than with the 3 mg. per kilogram dose. There was but little if any change in the respiratory rate following the ingestion of caffeine. In one subject there appeared to be a slight increase after the 6 mg. per kilogram dose. The tidal volume, however, showed a definite increase with the 3 mg. and a pronounced increase with the 6 mg. per kilogram dose. The depth of respiration was therefore increased by the larger doses of caffeine, evidently the result of a stimulation of the respiratory center. It is concluded that the rise in the respiratory quotient was due solely to the blowing off of preformed carbon dioxide. Hyperventilation in the first two 15-minute post-ingestion periods was followed by a compensatory retention of carbon dioxide which was completed at the conclusion of the experiment. The increase in metabolism produced by the larger doses of caffeine was not accompanied by a change in the proportion of the foodstuffs oxidized.