Hyperhydration with Glycerol, Sodium, and Isomaltulose or Sucrose on Fluid Balance, Thermoregulation, and Exercise Capacity in the Heat.

Purpose: We aimed to investigate whether the addition of carbohydrates (sucrose [Suc] or isomaltulose [Iso]) to a beverage containing glycerol (Gly) and sodium (Na) would enhance fluid balance, thermoregulatory response, and high-intensity exercise capacity during exercise in hot environments.

Methods: In a randomized, double-blinded, and crossover study, 13 healthy men consumed 1L of beverage containing (1) 7% Gly + 0.5% Na (Gly + Na), (2) Gly + Na with 7% Suc (Gly + Na + Suc), (3) Gly + Na with 7% Iso (Gly + Na + Iso), or (4) water (CON) over 40-min in a hot environment (32 °C, 50% relative humidity). Participants then completed three 30-min cycling bouts at 50% peak oxygen consumption (V̇O2peak), followed by a time-to-exhaustion (TTE) trial at 80% V̇O2peak. Fluid balance and thermoregulatory response were assessed throughout the experiment.

Results: Compared with CON, beverages containing Gly and Na attenuated the total urine volume (all P < 0.001), and the addition of Iso (331 ± 84 g) further encouraged this response compared with Gly + Na and Gly + Na + Suc (429 ± 68 and 445 ± 133 g, respectively, both P ≤ 0.030). Compared with CON, the plasma volume increased with Gly + Na + Suc during the first exercise bout (-2.2 ± 6.7 and 4.3 ± 5.4 %, respectively, P = 0.048) and with Gly + Na + Iso during the TTE (-9.1 ± 4.4 and - 4.1 ± 4.0 %, respectively, P = 0.025). The rectal temperature increased whereas local sweating responses were reduced more with Gly- and Na-containing beverages than with CON (all P ≤ 0.028). No differences were found in TTE among the beverages (P = 0.159).

Conclusions: Adding Suc or Iso to Gly- and Na-contained beverages partially improves fluid balance but does not improve thermoregulatory responses and performance during moderate-intensity exercise in hot environments.