SODIUM-INTAKE AND POSTEXERCISE REHYDRATION IN MAN

This study examined the effect of the sodium content of drinks on the rehydration process after exercise. Six healthy male volunteers were d ehydrated by a mean (SEM) of 1.9(0.0)% of body mass by intermittent cy cle exercise in a warm (32 degrees C), humid (54% RH) environment. Sub jects exercised on four occasions at weekly intervals with each trial beginning in the morning, 3 h after a standard breakfast. Over a 30-mi n period beginning 30 min after the end of exercise, subjects ingested one of the four test drinks in a volume equivalent to 1.5 times their body mass loss. Drink composition was constant except for the sodium (and matching anion) content. Sodium content of drinks A, B, C and D w as 2, 26, 52 and 100 mmol . l(-1), respectively. Treatment order was r andomised using a four-way crossover incomplete block design. Blood an d urine samples were obtained before exercise, immediately before and after the rehydration period and at 0.5, 1.5, 3.5 and 5.5 h after the end of the rehydration period. Data were analysed by parametric or non -parametric statistical tests as appropriate. The volume of fluid cons umed was the same on all trials [2045(45) ml]. From the 1.5-h sample o nwards, a significant treatment effect on cumulative urine output was apparent, with the volume excreted being inversely related to the sodi um content of the drink consumed. By the end of the trial, subjects we re in net negative fluid balance on trials A [by 689(124) ml] and B [b y 359(87) ml]; on trials C [-2(79) ml] and D [+98(67) ml], subjects we re approximately euhydrated. Cumulative urinary sodium output was high er on treatment D than on the other trials after 5.5 h. Plasma volume was lower after exercise than before; on trials B, C and D, plasma vol ume was higher than the pre-exercise value from 0.5 h after the end of the rehydration period onwards. On trial A, plasma Volume was higher than the preexercise value at 3.5 and 5.5 h after the end of the rehyd ration period. At 1.5 h after the end of the rehydration period, the i ncrease in plasma volume was greater on trials C and D than on trial A . These results suggest that the fraction of the ingested fluid that w as retained was directly related to the sodium concentration.