Author(s): HUNT, L., HUSTON, H., COOPER-SMITH, N., ARRELL, E., HARRIS, C., SUTTON, E., TYE, J., BLOW, A., SHEPHERD, S.O., Institution: PRECISION FUEL AND HYDRATION, Country: UNITED KINGDOM, Abstract-ID: 1109

INTRODUCTION:
Exercise-associated muscle cramps (EAMCs) can have race-ruining consequences for endurance athletes, though the relevance of sodium (Na+) loss to cramp risk remains debated. Previous laboratory and field studies reporting no differences in serum [Na+] between athletes who do and do not experience cramp have contributed to the view that electrolyte disturbance is not a key factor in cramp risk. However, serum [Na+] is tightly regulated, potentially obscuring meaningful inter-individual differences in Na+ loss during prolonged endurance exercise. Consequently, the relationship between sweat [Na+] and EAMC risk remains insufficiently characterised in large, real-world athlete populations. We, therefore, aimed to examine the association between sweat [Na+] and EAMC frequency, and to provide normative sweat [Na+] across a diverse population.
METHODS:
Sweat [Na+] and self-reported EAMC frequency data from 24,498 athletes (men: 18,924; women: 4,963; prefer not to say: 611) undergoing standardised sweat testing were analysed. Associations were assessed using the Kruskal-Wallis H test and ordinal logistic regression. Secondary analyses on a larger cohort (n = 29,835) across 32,249 individual tests examined relationships between sweat [Na+] and interactions with sex, age, competitive level (professional [n=4,876] and recreational [n= 27,373]) and sport category (endurance [n=16,742], team [n=1,002], other [n= 14,457]) without EAMC frequency data.
RESULTS:
Sweat Na+ concentration differed significantly across cramping categories (H = 293.06, p < 0.001), increasing from 948 mg·L-1 (never) to 1065 mg·L-1 (often). Sweat [Na+] concentration was a predictor of cramping frequency (OR = 1.18 per 250 mg·L-1 increase, 95% CI 1.16–1.20, p < 0.001). Athletes with sweat [Na+] of 1500 mg·L-1 had approximately twice the odds of reporting a higher cramping frequency (never to rarely, rarely to sometimes, sometimes to often, often to always) compared to those at 500 mg·L-1 (OR = 1.94). A significant sex × age interaction was observed (p < 0.001); women in older age groups exhibited higher sweat Na+, with differences most pronounced around menopausal age, while men showed stable values across age groups.
CONCLUSION:
These data demonstrate an elevated risk of EAMCs with increased sweat sodium concentration. While the modest effect size is consistent with the multifactorial aetiology of EAMC (namely, neuromuscular fatigue and free water loss), these findings suggest that higher individual sweat Na+ losses increase the risk of cramping. Individualising sodium replacement strategies beyond standardised guidelines for endurance athletes warrants further investigation. Additionally, the observed rise in sweat [Na+] in women around menopausal age highlights a need for further research into the potential hormonal and physiological mechanisms driving these differences.