Continuous assessment of the sweat rate-lactate content association during acetylcholine-induced sweating: an exploratory study.

PURPOSE: Sweat contains lactate; however, the sweat rate-sweat lactate content dynamic association remains unclear. We investigated the pattern of this association using a newly developed continuous measurement system under pharmacological cholinergic stimulation, across habitually-trained and untrained individuals. METHODS: Thirty healthy young adults (habitually trained, 15; untrained, 15) underwent 10% acetylcholine iontophoresis at two forearm sites, generating an initially high sweat rate followed by a gradual decline over 40 min. Sweat rate (ventilated capsule) and sweat lactate content (phosphate-buffered saline-perfused microfluidic lactate measurement system) were continuously measured. The sweat rate-sweat lactate content association was characterized using either simple or segmented linear regression, and the better-fitting model was identified. RESULTS: Segmented linear regression provided a better fit in 27 participants (trained, 13; untrained, 14). Among these participants, 20 (trained, 9; untrained, 11) demonstrated an inflection point where sweat lactate content more steeply increased once a sweat rate exceeded a certain level. This breakpoint occurred at a higher sweat rate in trained than that in untrained individuals (0.47 ± 0.18 vs. 0.26 ± 0.15 mg·cm- 2·min- 1, P = 0.016). A follow-up study revealed variability in the response-pattern classification, and the breakpoint sweat rate showed moderate reliability when the same increase-type pattern was observed across trials. CONCLUSION: These findings suggest that the sweat rate-sweat lactate content association can demonstrate nonlinear characteristics during cholinergic sweating. Among individuals exhibiting an increase-type breakpoint, trained participants demonstrated this breakpoint at a higher sweat rate. However, the physiological meaning of this breakpoint remains unclear.