Ad libitum water consumption off-sets the thermal and cardiovascular strain exacerbated by dehydration during a 3-h simulated heatwave.

Abstract

To assess whether ad libitum water ingestion of different temperatures is sufficient to prevent dehydration-related exacerbations of thermal and cardiovascular strain, during exposure to conditions representative of a heatwave. Twelve participants (mean ± SD; 25 ± 4years) exercised for 180min at 3 METs in 40.1 ± 0.6°C, 40.4 ± 2.1%RH four times: (i) consuming 20°C water ad libitum (AL20); (ii) consuming 4°C water ad libitum (AL4); (iii) replacing no fluids (NOFR); (iv) replacing sweat losses (FULLFR). Fluid consumption (FC), dehydration (%DEH), rectal temperature (Tre), rate-pressure product (RPP), forearm blood flow (FBF), mean skin temperature (Tsk), and local sweat rate (LSR) were measured/determined. FC was greater in AL20 (1.30 ± 0.41 L) than AL4 (1.03 ± 0.32 L; P = 0.003). %DEH was lower (P < 0.001) in AL20 (0.11 ± 0.76%), AL4 (0.43 ± 0.64%), and FULLFR (0.01 ± 0.12%) compared to NOFR (1.93 ± 0.28%). %DEH was lower in AL20 than AL4 (P = 0.003). In NOFR, end-trial changes in Tre were greater (P < 0.001) (1.05 ± 0.27°C) compared to all other trials, but similar among AL20 (0.72 ± 0.30°C), AL4 (0.76 ± 0.25°C) and FULLFR (0.74 ± 0.35°C). End-trial RPP was higher (P < 0.001) in NOFR (12,389 ± 1578mmHg·bpm) compared to all other trials, but similar among FULLFR (11,067 ± 1292mmHg·bpm), AL20 (11,214 ± 2078mmHg·bpm) and AL4 (11,089 ± 1795mmHg·bpm). No differences in Tsk or LSR were observed among trials, but FBF was lower in NOFR compared to FULLFR (2.84 ± 0.69 vs. 3.52 ± 0.96ml/100ml/min; P = 0.029). 4°C or 20°C ad libitum water ingestion prevented dehydration levels that exacerbate thermal/cardiovascular strain, despite blunted fluid intake with 4°C water. Higher core temperatures with NOFR are attributed to impaired internal heat distribution secondary to a lower FBF.