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Purpose <p>Hypohydration of ≥ 2% body mass loss (BML) can impair cycling performance in the heat; however, it remains unclear whether this is primarily due to the body water deficit itself or the concomitant, typically observed increase in serum sodium concentration (S<sub>[Na</sub><sup>+</sup><sub>]</sub>). To disentangle these effects, we examined the performance effects of 60% (60FR) vs. 80% (80FR) fluid replacement, designed to achieve &gt; 2% vs &lt; 2% BML, with personalised sodium prescription targeting condition-matched S<sub>[Na</sub><sup>+</sup><sub>]</sub>.</p> Methods <p>Eleven male cyclists (35 ± 6&#xa0;years, VȮ<sub>2max</sub> 58 ± 5&#xa0;ml·kg<sup>−1</sup>·min<sup>−1</sup>) completed two trials, consisting of 180&#xa0;min cycling (90% LT<sub>1</sub>, 32&#xa0;°C, 50% humidity), followed by a work-matched time-trial (TT). Participants received either 60% or 80% of projected fluid losses, along with sodium supplementation aiming to maintain S<sub>[Na</sub><sup>+</sup><sub>]</sub> where dilution of S<sub>[Na</sub><sup>+</sup><sub>]</sub> was predicted. Outcomes included BML, S<sub>[Na</sub><sup>+</sup><sub>]</sub>, plasma volume change (∆PV), rectal (T<sub>re</sub>) and skin (T<sub>sk</sub>) temperatures, whole-body sweat rate (WBSR), and TT performance.</p> Results <p>BML was greater in 60FR at all time-points (P &lt; 0.01), whereas S<sub>[[Na</sub><sup>+</sup><sub>]</sub> remained stable across both trials (P = 0.07). In 80FR, ∆PV was smaller during the preload, T<sub>re</sub> was lower from 120&#xa0;min onwards and T<sub>sk</sub> and WBSR were higher during the TT compared to 60FR (all P &lt; 0.05). TT completion time was 8 ± 10% faster in 80FR (1793 ± 174 vs 1968 ± 246&#xa0;s, P = 0.02).</p> Conclusion <p>Fluid replacement resulting in BML &gt; 2% impairs thermoregulation and cycling performance in the heat, compared to maintaining BML &lt; 2%, despite no differences in S<sub>[Na</sub><sup>+</sup><sub>]</sub>.</p>

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Inadequate fluid replacement during prolonged cycling in the heat impairs time trial performance independent of serum sodium concentrations

  • 期刊论文

  • 出版模式：

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  • 发表日期：

    2026年5月2日

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  • Sean A. Sage,
  • Fabiana I. Smaira,
  • Jeffrey W. F. Aldous,
  • Martyn G. Morris,
  • Andrew Blow,
  • Jonny Tye,
  • Michael L. Newell

摘要

Purpose

Hypohydration of ≥ 2% body mass loss (BML) can impair cycling performance in the heat; however, it remains unclear whether this is primarily due to the body water deficit itself or the concomitant, typically observed increase in serum sodium concentration (S[Na+]). To disentangle these effects, we examined the performance effects of 60% (60FR) vs. 80% (80FR) fluid replacement, designed to achieve > 2% vs < 2% BML, with personalised sodium prescription targeting condition-matched S[Na+].

Methods

Eleven male cyclists (35 ± 6 years, VȮ2max 58 ± 5 ml·kg−1·min−1) completed two trials, consisting of 180 min cycling (90% LT1, 32 °C, 50% humidity), followed by a work-matched time-trial (TT). Participants received either 60% or 80% of projected fluid losses, along with sodium supplementation aiming to maintain S[Na+] where dilution of S[Na+] was predicted. Outcomes included BML, S[Na+], plasma volume change (∆PV), rectal (Tre) and skin (Tsk) temperatures, whole-body sweat rate (WBSR), and TT performance.

Results

BML was greater in 60FR at all time-points (P < 0.01), whereas S[[Na+] remained stable across both trials (P = 0.07). In 80FR, ∆PV was smaller during the preload, Tre was lower from 120 min onwards and Tsk and WBSR were higher during the TT compared to 60FR (all P < 0.05). TT completion time was 8 ± 10% faster in 80FR (1793 ± 174 vs 1968 ± 246 s, P = 0.02).

Conclusion

Fluid replacement resulting in BML > 2% impairs thermoregulation and cycling performance in the heat, compared to maintaining BML < 2%, despite no differences in S[Na+].