Fluid and electrolyte balance in endurance athletes: from lab to field

Endurance athletes often experience disturbances in fluid and electrolyte balance that can impact performance and health. While pre-exercise hyperhydration and personalised in-race fluid and sodium intake may mitigate these effects, difficulties in practical implementation can limit their efficacy. Across this thesis, a series of laboratory and field studies examined sodium‐induced hyperhydration (SIH), self-selected strategies, personalised hydration, and blood sodium prediction across running and cycling exercise, strengthening the evidence base underpinning hydration guidance. Chapter 4 investigated how the concentration of SIH solutions influenced physiological responses and 5 km time-trial (TT) running performance in the heat. A high-sodium solution (120 mmol∙L-1) improved fluid retention (+~0.2 L) and running economy (RE; +~2.5%) relative to water, whereas a moderate-sodium solution (82 mmol∙L-1) did not. Both sodium solutions prevented the ~2 mmol∙L-1 reduction in serum sodium concentration (SNa) with water. Despite the hydration and RE benefits with the high-sodium solution, TT performance was not different between trials. Chapter 5 used a between-group design to evaluate the efficacy and feasibility of SIH (13 mL∙kg body mass (BM) -1, 120 mmol∙L-1 NaCl) compared with ad-libitum fluid and electrolyte intake (ALI) before a 10-mile running race. Participants consumed 99.5% of the SIH solution, resulting in increased pre-race fluid intake (+6.3 mL∙kg BM-1) and retention (+~0.6 L), but no change in blood electrolytes compared with ALI. SIH was perceived to provide a performance benefit but increased gastrointestinal symptoms, reduced drink palatability, and lowered ease of consumption, raising questions about the feasibility of athletes regularly adopting SIH. Chapter 6 assessed the effects of 60% (60FR) vs 80% (80FR) fluid replacement, with modelling-derived sodium prescription, targeting maintenance of SNa, on cycling performance in the heat. BM loss was greater in 60FR, notably at 180 min (-2.5 ± 0.5 vs -1.5 ± 0.5%), but SNa remained stable across both trials over time. Reduced hypovolaemia, lower rectal temperature, higher sweat rate and faster TT completion (8 ± 10%) in 80FR together indicate that inadequate fluid replacement impairs thermoregulation and cycling performance, independent of SNa. Chapter 7 assessed hydration behaviours and responses in a six-hour ultra-endurance running race and determined the field-based validity of the Kurtz-Nguyen blood sodium prediction equation. Despite all runners being in a whole-body sodium deficit, there were no cases of dysnatraemia. There was inter-individual variability in ∆BM (-4.8% to +0.4%), and better performance was associated with higher sweat rate and BML. The Kurtz-Nguyen showed minimal bias at a group level, but wide limits of agreement suggest it is unsuitable for individual predictions in the field. Findings from this thesis indicate that SIH with concentrations ≥120 mmol∙L-1 can effectively increase total body water, and may improve RE, but should be prescribed only after beingextensively trialled. Future research is warranted to explore beverage formulations that improve palatability. Regarding intra-exercise hydration, this thesis indicates that personalising fluid and sodium intake may benefit cycling performance in laboratory settings, but blood sodium prediction equations are unreliable in field-based settings on an individual level. Future research is therefore required before equations are widely used by practitioners.