Comparison of Resting Pilocarpine-Induced Sweat Sodium Concentration and Exercise-Induced Sweat Sodium Concentration

Abstract

INTRODUCTION: Resting sweat sodium concentration is commonly estimated using pilocarpine iontophoresis and a commercial sweat sodium analyzer (SSA), yet it may not reflect changes in sweat sodium during exercise. PURPOSE: To compare pilocarpine-induced (resting) sweat sodium concentration with exercise-induced sweat sodium concentration during a 90-minute treadmill run. METHODS: Twenty-five adults (19–67 y) completed 32 total trials. Before exercise, a resting sweat sample was induced on the upper forearm via pilocarpine iontophoresis and collected with a Macroduct® device, then analyzed on the SSA. Participants then ran for 90 minutes at moderate intensity (~65% heart-rate reserve). Exercise sweat was collected on the inner forearm with Macroduct® collectors that were replaced at 15–30 minutes and at 90 minutes and analyzed on the SSA. Participants were categorized into high- and moderate-fitness groups based on heart-rate reserve. RESULTS: Mean (±SD) resting pilocarpine-induced sodium concentration was 51±17 mM. Exercise-induced sodium concentration averaged 52±18 mM at 15–30 minutes and 57±27 mM at 90 minutes, with no statistically significant differences versus pilocarpine values (p=0.73 and p=0.11, respectively). Subgroup trends indicated greater divergence in moderately fit participants, particularly moderately fit females, who showed the largest average increase from pilocarpine values by 90 minutes (~20% higher). CONCLUSION: Although group means were similar at rest and during exercise, meaningful individual and subgroup variability was observed. Resting pilocarpine-induced sweat sodium may not reliably represent exercise sweat sodium for all athletes, especially moderately fit females, highlighting the need for exercise-specific measurements when developing hydration and electrolyte replacement strategies.