Effect of water ingestion on cardiovascular and thermal responses to prolonged cycling and running in humans: A comparison

The aim of this investigation was to examine the effect of water ingestion on physiological responses to prolonged cycling (CYC) and running (RUN). A group of 11 men with mean (SEM) maximal oxygen uptake (V̇O_2max) 48.5 (1.8) ml·kg^-1·min^-1 on a cycle-ergometer and 52.1 (2.2) ml·kg^-1·min^-1 on a treadmill (P < 0.01) exercised for 90 min on four occasions, twice on each ergometer, at 60% of mode specific V̇O_2max. No fluid was taken (D) in one trial on each ergometer, whereas 60% of fluid losses were replaced by drinking water in the other trial (W). In CYC, water ingestion attenuated the change in cardiac output (ΔQ̇_c) and the reduction in stroke volume (ΔSV) [ΔSV: -22.7 (3.8) in D, -10.7 (2.9) ml·beat ^-1 in W, P < 0.01; ΔQ̇_c: -1.9 (0.5) in D, -0.2 (0.4) l·min^-1 in W at 85 min, P < 0.01], but did not affect rectal temperature [T_re at 90 min: 38.8 (0.1)°C in D, 38.7 (0.1)°C in W]. In contrast, fluid replacement reduced hyperthermia in RUN [T_re at 90 min: 39.6 (0.2) in D, 39.1 (0.2)°C in W, P < 0.01], and this was linked with a higher skin blood flow [RUN-W 88.9 (8.5), RUN-D 70.7 (8.4)%, P < 0.05]. The ΔQ̇_c and ΔSV were also attenuated with water ingestion in this mode of exercise (P < 0.05). It is concluded that water ingestion improves physiological function in both cycling and running, but that the underlying mechanism is different in the two modes of exercise.