Do endurance athletes need repeat sweat sodium testing after heat acclimation? Separating physiological adaptation from measurement variability

Heat acclimation lowers the sodium concentration of sweat, a quantity widely measured to individualize fluid and electrolyte replacement in endurance athletes. Whether it should be repeated during or after acclimation remains unsettled. This Mini Review argues that the question is not whether the adaptation is real, since it reproduces across protocols and decades, but whether a second measurement can resolve it. Studies place the effect between roughly 9 and 23 mmol/L, or approximately 15 mmol/L at a matched local sweat rate, with a protocol-dependent time course: conservation detectable from the third day, before sweat rate rises, plateauing by the sixth day in field conditions but still falling at 10 days under laboratory acclimation. Against that signal, raising exercise intensity from 45% to 65% of maximal oxygen uptake shifts whole-body sweat sodium concentration by approximately 20 mmol/L, while collection site, technique, sample handling, and sodium intake add further variance. Analytical instrumentation is not the limiting step; procedural and biological variation is. Two interpretations follow: one treats serial testing as necessary to track a fast, heterogeneous adaptation, the other notes that the concentration predicts neither hydration status nor sweating rate, does not by itself stratify hyponatremia risk, and so rarely changes a decision. They differ in the quantity they estimate, not in their reading of the evidence. Repeat testing is defensible only under a reproduced heat stress protocol, reported as whole-body sodium loss rate, and interpreted against a smallest detectable change not yet established for acclimation monitoring; retest intervals remain hypotheses.