Purpose: This study compared the voluntary fluid intake behavior of older men and women (54-70 yr) when provided cold, palatable beverages and ample opportunity to drink between repeated bouts of exercise in the heat.
Methods: Thirteen men and 14 women performed four bouts of 15-min cycling at 65% V̇O2peak followed by 15 min of rest at 30°C and 50% relative humidity. In separate trials, subjects drank either a carbohydrate-electrolyte solution (CES) or water ad libitum during the rest periods and were unaware that their fluid intake was being measured.
Results: Fluid intake behavior was repeatable (intraclass correlation coefficient = 0.75), and subjects drank enough of either beverage to match sweating rates and maintain their body mass (BM). Fluid intake per kilogram of BM was greater with CES (18.7 ± 2.2 vs 15.1 ± 2.1 mL·kg-1; P < 0.05), and plasma volume (PV) was better maintained during the CES trials (-1.3 ± 1.1 vs -4.2 ± 1.1% during the second half of the session). Women drank significantly more water than the men on a per kilogram basis (17.2 ± 2.9 vs 12.8 ± 1.7 mL·kg-1 BM), and one woman (BM = 45.7 kg) became hyponatremic (SNA = 126 mmol·L-1) with symptoms during the water trial.
Conclusion: Older adults drank enough to maintain fluid balance when palatable fluid was readily available; however, CES promoted greater voluntary fluid intake and restored PV losses faster than water. In addition, older women drank more water than men during interval exercise in the heat, which may put smaller women at an increased risk for developing hyponatremia.