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Effect of Permissive Dehydration on Induction and Decay of Heat Acclimation, and Temperate Exercise Performance

View Article: PubMed Central - PubMed

ABSTRACT

Purpose:: It has been suggested that dehydration is an independent stimulus for heat acclimation (HA), possibly through influencing fluid-regulation mechanisms and increasing plasma volume (PV) expansion. There is also some evidence that HA may be ergogenic in temperate conditions and that this may be linked to PV expansion. We investigated: (i) the influence of dehydration on the time-course of acquisition and decay of HA; (ii) whether dehydration augmented any ergogenic benefits in temperate conditions, particularly those related to PV expansion.

Methods: : Eight males [VO2max: 56.9(7.2) mL·kg−1·min−1] undertook two HA programmes (balanced cross-over design), once drinking to maintain euhydration (HAEu) and once with restricted fluid-intake (HADe). Days 1, 6, 11, and 18 were 60 min exercise-heat stress tests [HST (40°C; 50% RH)], days 2–5 and 7–10 were 90 min, isothermal-strain (Tre ~ 38.5°C), exercise-heat sessions. Performance parameters [VO2max, lactate threshold, efficiency, peak power output (PPO)] were determined pre and post HA by graded exercise test (22°C; 55%RH).

Results: : During isothermal-strain sessions hypohydration was achieved in HADe and euhydration maintained in HAEu [average body mass loss −2.71(0.82)% vs. −0.56(0.73)%, P < 0.001], but aldosterone concentration, power output, and cardiovascular strain were unaffected by dehydration. HA was evident on day 6 {reduced end-exercise Tre [−0.30(0.27)°C] and exercise heart rate [−12(15) beats.min−1], increased PV [+7.2(6.4)%] and sweat-loss [+0.25(0.22) L.h−1], P < 0.05} with some further adaptations on day 11 {further reduced end-exercise Tre [−0.25(0.19)°C] and exercise heart rate [−3(9) beats.min−1], P < 0.05}. These adaptations were not notably affected by dehydration and were generally maintained 7-days post HA. Performance parameters were unchanged, apart from increased PPO (+16(20) W, irrespective of condition).

Conclusions: : When thermal-strain is matched, permissive dehydration which induces a mild, transient, hypohydration does not affect the acquisition and decay of HA, or endurance performance parameters. Irrespective of hydration, trained individuals require >5 days to optimize HA.

No MeSH data available.


Related in: MedlinePlus

Mean(SD) results from temperate (22°C, 55% RH) graded exercise test performed Pre- and Post- heat acclimation, with (HADe: black) and without (HAEu gray) permissive dehydration (n = 8). (A) Lactate Threshold; (B) Gross Mechanical Efficiency (GME); (C) Maximal Oxygen Uptake (VO2max); (D) Peak Power Output (PPO). * = Significant main effect for time (P < 0.05).
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Figure 3: Mean(SD) results from temperate (22°C, 55% RH) graded exercise test performed Pre- and Post- heat acclimation, with (HADe: black) and without (HAEu gray) permissive dehydration (n = 8). (A) Lactate Threshold; (B) Gross Mechanical Efficiency (GME); (C) Maximal Oxygen Uptake (VO2max); (D) Peak Power Output (PPO). * = Significant main effect for time (P < 0.05).

Mentions: Ambient conditions for the GXT were 22.0(0.2)°C, 54.6(5)% RH. Both of the heat acclimation programmes reduced the thermo-physiological burden under temperate conditions, as evidenced by a significant time effect (GXTpre vs. GXTpost) for resting and exercise Tre and heart rate, end exercise (all reduced), and skin blood flow (increased). The only significant condition effect was for RER, which was higher in HAEu than HADe, but there were no significant interaction effects [see Table 3 (Supplementary Material)]. With regard to parameters related to endurance performance, there were no significant main effects for time or condition, or the time × condition interaction for VO2max, lactate threshold or GME (see Figure 3). There was a significant main effect of time on PPO achieved during the GXT (P = 0.033), but the condition and interaction effects were not significant (see Figure 3) and the increase in PPO was not correlated with any of the improvements in thermoregulatory function. Likewise, maximum heart rate (fCmax) reached in the GXT was significantly reduced following HA [from 187(7)b·min−1 to 183(7) beats·min−1 in HAEu and from 189(10) to 181(9) beats·min−1 in HADe, P = 0.003] but, the condition and interaction effects were, again, not significant.


Effect of Permissive Dehydration on Induction and Decay of Heat Acclimation, and Temperate Exercise Performance
Mean(SD) results from temperate (22°C, 55% RH) graded exercise test performed Pre- and Post- heat acclimation, with (HADe: black) and without (HAEu gray) permissive dehydration (n = 8). (A) Lactate Threshold; (B) Gross Mechanical Efficiency (GME); (C) Maximal Oxygen Uptake (VO2max); (D) Peak Power Output (PPO). * = Significant main effect for time (P < 0.05).
© Copyright Policy
Related In: Results  -  Collection

License
Show All Figures
getmorefigures.php?uid=PMC5120118&req=5

Figure 3: Mean(SD) results from temperate (22°C, 55% RH) graded exercise test performed Pre- and Post- heat acclimation, with (HADe: black) and without (HAEu gray) permissive dehydration (n = 8). (A) Lactate Threshold; (B) Gross Mechanical Efficiency (GME); (C) Maximal Oxygen Uptake (VO2max); (D) Peak Power Output (PPO). * = Significant main effect for time (P < 0.05).
Mentions: Ambient conditions for the GXT were 22.0(0.2)°C, 54.6(5)% RH. Both of the heat acclimation programmes reduced the thermo-physiological burden under temperate conditions, as evidenced by a significant time effect (GXTpre vs. GXTpost) for resting and exercise Tre and heart rate, end exercise (all reduced), and skin blood flow (increased). The only significant condition effect was for RER, which was higher in HAEu than HADe, but there were no significant interaction effects [see Table 3 (Supplementary Material)]. With regard to parameters related to endurance performance, there were no significant main effects for time or condition, or the time × condition interaction for VO2max, lactate threshold or GME (see Figure 3). There was a significant main effect of time on PPO achieved during the GXT (P = 0.033), but the condition and interaction effects were not significant (see Figure 3) and the increase in PPO was not correlated with any of the improvements in thermoregulatory function. Likewise, maximum heart rate (fCmax) reached in the GXT was significantly reduced following HA [from 187(7)b·min−1 to 183(7) beats·min−1 in HAEu and from 189(10) to 181(9) beats·min−1 in HADe, P = 0.003] but, the condition and interaction effects were, again, not significant.

View Article: PubMed Central - PubMed

ABSTRACT

Purpose:: It has been suggested that dehydration is an independent stimulus for heat acclimation (HA), possibly through influencing fluid-regulation mechanisms and increasing plasma volume (PV) expansion. There is also some evidence that HA may be ergogenic in temperate conditions and that this may be linked to PV expansion. We investigated: (i) the influence of dehydration on the time-course of acquisition and decay of HA; (ii) whether dehydration augmented any ergogenic benefits in temperate conditions, particularly those related to PV expansion.

Methods: : Eight males [VO2max: 56.9(7.2) mL&middot;kg&minus;1&middot;min&minus;1] undertook two HA programmes (balanced cross-over design), once drinking to maintain euhydration (HAEu) and once with restricted fluid-intake (HADe). Days 1, 6, 11, and 18 were 60 min exercise-heat stress tests [HST (40&deg;C; 50% RH)], days 2&ndash;5 and 7&ndash;10 were 90 min, isothermal-strain (Tre ~ 38.5&deg;C), exercise-heat sessions. Performance parameters [VO2max, lactate threshold, efficiency, peak power output (PPO)] were determined pre and post HA by graded exercise test (22&deg;C; 55%RH).

Results: : During isothermal-strain sessions hypohydration was achieved in HADe and euhydration maintained in HAEu [average body mass loss &minus;2.71(0.82)% vs. &minus;0.56(0.73)%, P &lt; 0.001], but aldosterone concentration, power output, and cardiovascular strain were unaffected by dehydration. HA was evident on day 6 {reduced end-exercise Tre [&minus;0.30(0.27)&deg;C] and exercise heart rate [&minus;12(15) beats.min&minus;1], increased PV [+7.2(6.4)%] and sweat-loss [+0.25(0.22) L.h&minus;1], P &lt; 0.05} with some further adaptations on day 11 {further reduced end-exercise Tre [&minus;0.25(0.19)&deg;C] and exercise heart rate [&minus;3(9) beats.min&minus;1], P &lt; 0.05}. These adaptations were not notably affected by dehydration and were generally maintained 7-days post HA. Performance parameters were unchanged, apart from increased PPO (+16(20) W, irrespective of condition).

Conclusions: : When thermal-strain is matched, permissive dehydration which induces a mild, transient, hypohydration does not affect the acquisition and decay of HA, or endurance performance parameters. Irrespective of hydration, trained individuals require &gt;5 days to optimize HA.

No MeSH data available.


Related in: MedlinePlus