Immediate Re-Hydration Post-Exercise is Not Coincident with Raised Mean Arterial Pressure Over A 30-Minute...

This investigation assessed the effects of immediate or delayed re-hydration post-exercise, on mean arterial blood pressure (MAP) and on blood plasma volume (PV) expansion post-exercise. It was hypothesised that fluid ingestion would raise MAP and attenuate PV expansion. On two occasions separated by seven days, eight males (age 20.4 ± 1.7 years, mass 79 ± 5 kg [means ± SD]; VO2max 48 ± 11 mL·kg(-1)·minute(-1), [mean ± SE]) cycled in the heat (35°C, 50% relative humidity) at a power output associated with 50% VO2max, until 1.0kg body mass was lost. 1L water was given either immediately thereafter, or two hours post-exercise by random assignment. On both occasions, MAP was calculated every five minutes for a period of 30-minutes post-exercise, and change in PV was calculated 24-hours post-exercise. Repeated measures ANOVA for MAP results suggested a low probability of a treatment effect (p = 0.655), a high probability of a time effect (p = 0.006), and a moderately high probability of a time x treatment interaction (p = 0.076); MAP tended to be lower when fluid had been consumed. PV expansions 24-hours post-exercise were not significant changes with respect to zero, and were not significantly different by treatment condition.

(a) The exercise was not sufficient to elicit significant PV expansions; thus, we were unable to determine the effects of the timing of post-exercise re-hydration on PV expansion. (b) The hypothesis regarding MAP in response to drinking was not supported, rather there was a 92% probability that the inverse affect occurs. Key PointsPost exercise hypotension is perhaps the most important mediator of plasma volume expansion post exerciseIt was hypothesised that drinking water immediately post exercise would attenuate post exercise hypotension by rapidly ameliorating dehydrationWe found that not only was our hypothesis incorrect, but rather a 92% probability exists that the inverse is true, i.e. drinking water in fact leads to lowered blood pressure, as compared to not drinking.