VOLUME REPLETION AFTER EXERCISE-INDUCED VOLUME DEPLETION IN HUMANS - REPLACEMENT OF WATER AND SODIUM LOSSES

Sodium and water loss during, and replacement after, exercise-induced volume depletion was investigated in six volunteers Volume depleted by 1.89 +/- 0.17% (SD) of body mass by intermittent exercise in a warm, humid environment. Subjects exercised in a large, open plastic bag, al lowing collection of all sweat secreted during exercise. For over 60 m in beginning 40 min after the end of exercise, subjects ingested drink s containing 0, 25, 50, or 100 mmol/l. sodium (trials 0, 25, 50, and 1 00) in a volume (ml) equivalent to 150% of the mass lost (g) by volume depletion. Body mass loss and sweat electrolyte (Na+, K+, and Cl-) lo ss were the same on each trial. The measured sweat sodium concentratio n was 49.2 +/- 18.5 mmol/l, and the total loss (63.9 +/- 38.7 mmol) wa s greater than that ingested on trials 0 and 25. Urine production over the 6-h recovery period was inversely related to the amount of sodium ingested. Subjects were in whole body negative sodium balance on tria ls 0 (-104 +/- 48 mmol) and 25 (-65 +/- 30 mmol) and essentially in ba lance on trial 50 (-13 +/- 29 mmol) but were in positive sodium balanc e on trial 100 (75 +/- 40 mmol). Only on trial 100 were subjects in po sitive fluid balance at the end of the study. There was a large urinar y loss of potassium over the recovery period on trial 100, despite a n egligible intake during volume repletion. These results confirm the im portance of replacement of sodium as well as water for volume repletio n after sweat loss. The sodium intake on trial 100 was appropriate for acute fluid balance restoration, but its consequences for potassium l evels must be considered to be undesirable in terms of whole body elec trolyte homeostasis for anything other than the short term.