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

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+].