Author(s): SAGE, S., HILL, L., ALDOUS, J., MORRIS, M., HUNT, L., NEWELL, M. , Institution: UNIVERSITY OF BEDFORDSHIRE, Country: UNITED KINGDOM, Abstract-ID: 2443

INTRODUCTION:
Sodium-induced pre-exercise hyperhydration can promote fluid retention, reduce cardiovascular and thermal strain, and improve cycling performance in the heat, yet evidence for its effects on time-trial (TT) running performance remains limited. Prior research has focused on high doses of sodium, yet it is uncertain whether moderate doses have similar physiological and performance outcomes. Furthermore, whilst emerging evidence indicates that glycerol-induced hyperhydration can improve running economy (RE), the impact of sodium-induced hyperhydration on RE remains unclear.
METHODS:
Ten non-heat-acclimated male runners (age 34 ± 8 years, body mass (BM) 76.9 ± 9.3 kg, V̇O₂ₘₐₓ 56 ± 5 ml·kg⁻¹·min⁻¹) completed three trials in a controlled hot environment (30 °C, 50% humidity, 2.9 m·s⁻¹ airflow). Each trial consisted of 60 min running at 90% of the first lactate threshold, followed by a 5-kilometre TT. Participants ingested 13 mL·kg BM⁻¹ of either water, an 82 mmol·L⁻¹ (ModNa⁺) or a 120 mmol·L⁻¹ (HighNa⁺) sodium chloride (NaCl) solution over a 60 min period, commencing 90 min before and ending 30 min prior to running. Linear mixed models were used to compare between trials, with Sidak-adjusted post hoc tests used to determine differences between trial and/or time where significant main or interaction effects were observed.
RESULTS:
Fluid retention (+0.205 L, 95% CI: +0.055 to +0.356 L, P = 0.006) and change in plasma volume from baseline (+5.3%, 95% CI: +1.1 to +9.6%, P = 0.004) were higher in HighNa⁺ compared with water, whereas ModNa⁺ did not produce the same effect (P ≥ 0.16). Serum sodium concentration (S[Na⁺]) decreased by 2.06 mmol·L⁻¹ following water ingestion (95% CI: −2.93 to −1.18 mmol·L⁻¹, P < 0.001), but both sodium-containing solutions prevented this serum dilution (P ≥ 0.55). RE during steady-state exercise was improved by ~3.0% in HighNa⁺ compared to water (−5.79 mL·kg⁻¹·km⁻¹, 95% CI: −9.96 to −1.62 mL·kg⁻¹·km⁻¹, P = 0.004), but ModNa⁺ did not elicit the same improvements (P = 0.19). TT performance was not different between trials (Water 19:02 ± 01:48 min, ModNa⁺ 18:44 ± 01:33 min, HighNa⁺ 18:52 ± 01:55 min, P = 0.31). Heart rate, rectal temperature, skin temperature, whole-body sweat rate, local sweat rate and sweat [Na⁺] were also not different between trials (all P > 0.05).
CONCLUSION:
Pre-exercise ingestion of 13 mL·kg BM⁻¹ of a 120 mmol·L⁻¹ NaCl solution maintained S[Na⁺] and improved hydration status and RE compared with water, whereas ingestion of an 82 mmol·L⁻¹ NaCl solution maintained serum electrolytes but did not enhance hydration status or RE. Neither sodium-containing solution improved 5 km TT performance or attenuated thermal strain compared with water. Overall, although higher drink Na⁺ concentrations more effectively supported fluid retention and reduced the oxygen cost of submaximal running, these physiological benefits did not translate into improved 5-kilometre TT performance.