The purpose of this investigation was to determine whether the increase in plasma volume (PV) frequently observed 24 hours after exercise is proportional to the magnitude of dehydration occurring during exercise. Seven males (age 21.6 ± 4.4 y, body mass 71.5 ± 8.5 kg; VO_2peak 43 ± 7 mL.kg.minute^-1, peak 60-second cycling power output 282 ± 16 W) completed three cycling sessions at 50% of peak power output in an ambient environment of 35oC, 50% relative humidity; with the exercise lasting either 30, 60 or 90 minutes (in random order) to elicit varying levels of dehydration (assessed by body mass changes). The percent change in PV was calculated 24 hours after each exercise session. All exercise sessions were separated by 7-days. Participants' body mass (means ± SD) decreased by 1.03 ± 0.22% in the 30-minute exercise protocol; 1.43 ± 0.26% in the 60-minute protocol; and 1.59 ± 0.37% in the 90-minute protocol. Significant PV expansions were not evident 24 hours after any protocol (0.76 ± 4.58% in the 30-minute protocol; 1.40 ± 4.58% in the 60-minute protocol, and 2.92 ± 3.2% in the 90-minute protocol). Regression analysis revealed a poor correlation between percent dehydration and percent change in plasma volume (r = 0.24). Our study revealed that the magnitude of dehydration elicited during this study was insufficient to stimulate a significant expansion in PV.

KEY WORDS: Exercise, dehydration, fluid volume, blood volume.