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

Abstract

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)°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⁻ K⁺, 12 mmol · l⁻¹ Cl⁻¹) 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⁺, Ku⁺ 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.