Plasma volume expansion 24-hours post-exercise: effect of doubling the volume of replacement fluid

The effects of two volumes (1.5 L or 3.0 L) of commercially available electrolyte beverage (1.44 mM·L(-1) Na(+)) taken during a 24-hour recovery period post-exercise, on plasma volume (PV) expansion 24-hours post-exercise were assessed. A simple random-order crossover research design was used. Subjects (n = 9 males: age 21 ± 4 years, body mass 80.0 ± 9.0 kg, peak incremental 60-second cycling power output 297 ± 45 W [means ± SD]) completed an identical exercise protocol conducted in hot ambient conditions (35(o)C, 50% relative humidity) on two occasions; separated by 7-days. On each occasion, subjects received a different volume of 24-hour fluid intake (commercial beverage) in random order. In each case, the fluid was taken in five equal aliquots over 24-hours. PV expansions 24-hours post-exercise were estimated from changes in haemoglobin and haematocrit. Dependent t-testing revealed no significant differences in PV expansions between trials, however a significant expansion with respect to zero was identified in the 3.0 L trial only. Specifically, PV expansions (%) were; 1.5 L trial: (mean ± SE) 2.3 ± 2.0 (not significant with respect to zero), 3.0 L trial: 5.0 ± 2.0 (p < 0.05, with respect to zero). Under the conditions imposed in the current study, ingesting the greater volume of the beverage lead to larger mean PV expansion. Key PointsGreater volume of re-hydration beverage is beneficial to mean PV expansion 3-hours post-exercise, however this relationship has not been previously tested under adequate controls over 24-hours to our knowledge.This study indicates that under the conditions we imposed, over 24-hours increased volume (1.5 Vs. 3.0 L) of oral re-hydration fluid is associated with increased mean PV expansion.Although we used an almost identical exercise stimulus as previous researchers, relative intensity was slightly lower in the current study. Coincidentally, PV expansions noted during this study were approximately half those reported by others.