Inadequate fluid replacement during prolonged cycling in the heat reduces performance independent of serum sodium concentrations

Hypohydration (≥2% body mass (BM) loss) impairs endurance performance and tends to result in both reduced total body water (TBW), and increased serum [Na+]. This study assessed the independent effect of TBW on cycling performance in the heat, by personalising Na+ and fluid ingestion to match serum [Na+] between conditions. Eleven male cyclists (35.0 ± 6.4 years, BM 76.0 ± 6.6 kg, lactate threshold (LT) 195 ± 20 W, VO˙2max 58 ± 5 ml·kg-1·min-1) were assessed for VO˙2max and LT (visit 1). In visit 2, participants completed 60 min of constant-load cycling (90% LT, 32°C, 50% rH) to determine sweat rate (1.2 ± 0.3 L.h-1) and electrolyte concentrations ([Na+] 46 ± 19 mmol.L-1, [K+] 4.0 ± 0.5 mmol.L-1). Post-exercise urine volume and electrolyte content were added to sweat losses to determine total losses. Experimental trials consisted of 180 min constant-load cycling (90% LT, 32°C, 50% rH), followed by a work-matched time-trial (TT). Baseline TBW was determined via bioelectrical impedance, with deuterium dilution-corrected regressions applied. During the 180 min bout, participants consumed 60% (60FR; 0.844 ± 0.202 L.h-1) or 80% (80FR; 1.125 ± 0.269 L.h-1) of projected total fluid losses. In each condition, participants consumed NaCl in capsules at a rate to maintain serum [Na+] (29.7 ± 26.3 mmol.h-1 in 80FR vs 4.3 ± 10.5 mmol.h-1 in 60FR, P = 0.003), based on projected losses and modelling equations (McCubbin, 2022), assuming baseline serum [Na+] was 140 mmol.L-1. TT completion time was 8 ± 10 % faster in 80FR (1793 ± 174 seconds vs 1968 ± 246 seconds, P = 0.023), with higher power output (254 ± 33 W vs 233 ± 32 W, P = 0.012), despite stable serum [Na+] across conditions (P=0.07). At 180 min, serum [Na+] was 139.37 ± 2.01 mmol.L-1 in 80FR vs 139.96 ± 2.28 mmol.L-1 in 60FR. BM loss was greater in 60FR at all time-points (P<0.05), including 180 min (2.5 ± 0.5 vs 1.5 ± 0.5 %) and post-TT (2.6 ± 0.9 vs 1.7 ± 0.8 %). TBW loss was greater in 60FR at all time-points (P<0.05), notably at 180 min (3.2 ± 0.8 vs 1.5 ± 0.9 %) and post-TT (3.3 ± 1.4 vs 1.9 ± 1.3 %). Results suggest reduced BM (>2%) and TBW (>3%) impair cycling performance in the heat, compared to maintaining losses within 2%, despite no difference in serum [Na+] between conditions. The data support individualising hydration strategies to mitigate fluid losses and ensure athletes can perform optimally in the heat.