Effects of acute hypobaric hypoxia on the appearance of ingested deuteriumfrom a deuterium oxide-labelled carbohydrate beverage in body fluids of humans during prolonged cycling exercise

To determine whether or not acute hypobaric hypoxia alters the rate of wate r absorption from a carbohydrate beverage ingested during exercise, six men cycled for 80 min on three randomly assigned different occasions. In one t rial, exercise was performed in hypoxia (barometric pressure, P-B = 594 hPa , altitude 4 400 m) at an exercise intensity selected to elicit 75% of the individual's maximal oxygen uptake (VO2max) previously determined in such c onditions. In the two other experiments, the subjects cycled in normoxia (P -B = 992 hPa) at the same absolute and the same relative intensities as in hypoxia, which corresponded to 55% and 75%, respectively, of their VO2max d etermined in normoxia. The subjects consumed 400 mi of a 12.5% glucose beve rage just prior to exercise, and 250 mi of the same drink at 20, 40 and 60 min from the beginning of exercise. The first drink contained 20 ml of deut erium oxide to serve as a tracer for the entry of water into body fluids. T he heart rate (HR) during exercise was higher in hypoxia than in normoxia a t the same absolute exercise intensity, whereas it was similar to HR measur ed in normoxia at the same relative exercise intensity. Both in normoxia an d hypoxia, plasma noradrenaline concentrations were related to the relative exercise intensity up to 40 min of exercise. Beyond that duration, when ex ercise was performed at the highest absolute power in normoxia, the noradre naline response was higher than in hypoxia at the same relative exercise in tensity. No significant differences were observed among experimental condit ions, either in temporal profiles of plasma D accumulation or in eliminatio n of water ingested in sweat. Conversely, elimination in urine of the water ingested appeared to be related to the severity of exercise, either high a bsolute power or the same relative power combined with hypoxia. We conclude d that water absorption into blood after drinking a 12.5% glucose beverage is not altered during cycling exercise in acute hypobaric hypoxia. It is su ggested that the elimination of water ingested in sweat and urine may be de pendent on local circulatory adjustments during exercise.