60 Vs. 80% Fluid Replacement With Personalised Sodium Ingestion: Impact On Thermoregulation And Cycling Performance

PURPOSE: Mathematical models estimate the sodium (Na+) intake required for maintaining serum [Na+] (McCubbin, 2022), and fluid replacement strategies can alter these requirements. We examined the effects of 60% (60FR) vs. 80% (80FR) fluid replacement, with personalised Na+ prescription targeting condition-matched serum [Na+], on thermoregulation and performance during prolonged cycling in the heat. METHODS: Male cyclists (n = 11, 35 ± 6 years, 76 ± 7 kg, V̇O2max 58 ± 5 ml·kg-1·min-1) first completed 60 min cycling (90% LT1, 32 °C, 50% RH), to determine whole-body sweat rate (WBSR) (1.2 ± 0.3 L.h-1) and sweat [Na+] (46 ± 19 mmol.L-1). Participants then performed two trials with a 180 min pre-load cycling bout (90% LT1, 32 °C, 50% RH), followed by a work-matched time-trial (TT). Participants ingested water to replace either 60 or 80% of projected total fluid losses, as well as a calculated Na+ dose to maintain serum [Na+]. Outcomes included body mass (BM) loss, rectal (Tre) and skin temperatures (Tsk), plasma volume changes (∆PV), WBSR, local sweat rate (LSR), sweat [Na+], and TT performance. Predicted (Kurtz & Nguyen, 2003) vs. measured serum [Na+] was compared post-preload for both trials. RESULTS: By design, fluid and Na+ ingestion rates were higher with 80FR (1.13 ± 0.27 vs 0.84 ± 0.20 L·h-1, and 29.7 ± 26.3 vs 4.3 ± 10.5 mmol·h-1). BM loss was greater in 60FR at all time-points (P < 0.01), notably at 180 min (2.5 ± 0.5 vs 1.5 ± 0.5 %), and post TT (2.6 ± 0.9 vs 1.7 ± 0.8 %). Serum [Na+] remained stable across both conditions over time (P = 0.07), but was higher post-preload than was predicted during both trials (139.3 ± 2.0 vs 138.3 ± 2.4 mmol.L-1 in 80FR and 139.5 ± 1.7 vs 138.1 ± 1.9 mmol.L-1 in 60FR, P < 0.02). Tre was lower, Tsk higher and ∆PV smaller in 80FR (all P < 0.05). No differences were observed for LSR, WBSR, or sweat [Na+] (all P > 0.05). TT completion time was 8 ± 10 % faster in 80FR (1793 ± 174 vs 1968 ± 246 sec, P = 0.02). CONCLUSION: Limiting fluid losses to <2% BM enhances thermoregulation and performance, even with stable serum [Na+] across conditions. Measured serum [Na+] exceeded predictions, highlighting a need for further investigation with sodium prescription. Together, these data support the use of personalised hydration strategies that minimise hypohydration and optimise exercise outcomes through improved thermoregulation in the heat.