Restoration of fluid balance after exercise-induced dehydration: Effects of food and fluid intake

This study investigated the effects of post-exercise rehydration with fluid alone or with a meal plus fluid. Eight healthy volunteers (five men, three women) were dehydrated by a mean of 2.1 (SEM 0.0)% of body mass by intermittent cycle exercise in a warm [34 (SEM 0)degrees C], humid [55 (SEM 1)% relative humidity] environment. Over 60 min beginning 30 min after exercise, the subjects ingested a commercially-available sports drink (21 mmol . l(-1) Na+, 3.4 mmol . l(-1) K+, 12 mmol . l(-1) Cl-) on trials A and B; on trial C a standard meal [63 kJ . kg(-1) body mass (53% CHO, 28% fat, 19% protein; 0.118 mmol . kJ(-1) Na+, 0.061 mmol . kJ(-1) K+)] plus drink (1 mmol . l(-1) Na+, 0.4 mmol . l(-1) K+, 1 mmol . l(-1) Cl-) were consumed. Water intake (in millilitres) was 150% of the mass loss (in grams). The trials took place after an overnight fast and were separated by 7 days. Blood and urine samples were collected at intervals throughout the study. Blood was analysed for haematocrit, haemoglobin concentration, serum osmolality, Na+, K+ and Cl- concentrations and plasma angiotensin II concentration. Urine volume, osmolality and electrolyte concentrations were measured. Dehydration resulted in a mean 5.2 (SEM 1.3)% reduction in plasma volume. With the exception of serum osmolality, which was higher on trial B than A at the end of the rehydration period, no differences were recorded for any of the measured parameters between trials A and B. Cumulative urine output following rehydration was lower (P < 0.01) on trial C [median 665 (range 396-1190)ml] than on trial B [median 934 (range 550-1403)ml] which was not different (P = 0.44) from trial A [median 954 (range 474-1501) ml]. Less urine was produced over the 1-h period ending 2 h after rehydration on trial C than on B (P = 0.01). On trials A and B the subjects were in net negative fluid balance by 337 (range 779-minus 306) ml and 373 (range 680-minus 173) ml, respectively (P < 0.01): on trial C the subjects were no different from their initial euhydrated state [median minus 29 (range minus 421-137) ml] 6 h after the end of rehydration (P = 1.00). A larger fraction of total water intake was retained when the standard meal plus drink was consumed. This may have been due to the larger quantities of Na+ and K+ ingested with the meal [mean 63 (SEM 4) mmol Na+, 21.3 (SEM 1.3) mmol K+] than with the drink [mean 42 (SEM 2) mmol Na+, 6.8 (SEM 0.4) mmol K+]. There was no difference between trials B and C in any of the measured blood parameters, but urinary Na+ and K+ excretion were both higher on trial C than B. These results suggest that post-exercise fluid replacement can be achieved by ingestion of water if consumed in sufficient volume together with a meal providing significant amounts of electrolytes.