EFFECT OF INGESTING VARYING CONCENTRATIONS OF SODIUM ON FLUID BALANCE DURING EXERCISE 350

Abstract

This study determined the effects of ingesting varying concentrations of Na+Cl- on fluid balance during 4 h of moderate-intensity (55% of VO2peak) exercise in mild (20 ±1°C, 70 ±5% r.h. [mean±SEM]) ambient conditions. In random order, 6 subjects (VO2peak 4.84 ±0.16 l·min-1) completed 8 × 25 min cycle bouts (5 min rest) while ingesting low (4.6 mEq·l-1; LS), moderate (50 mEq·l-1; MS) or high (100 mEq·l-1; HS) amounts of Na+cl- as gel capsules, with an 8% carbohydrate(CHO) solution. Subjects consumed 400 ml before & a capsule in 150 ml·10 min-1 during exercise. Heart rate & rectal temperature increased to a similar extent from 15 to 240 min in all three trials(132±1 to 140 ±1 beat·min-1; 37.8 ±0.1 to 38.5 ±0.1 °C). Sweat rate (0.98 ±0.06 vs. 0.91 ± 0.06 vs. 0.89 ±0.06 l·h-1) & Na+ sweat losses (47±2 vs. 40 ±2 vs. 36 ±1 mEq·h-1 for LS, MS and HS) were not different between treatments. Plasma [Na+] remained stable throughout exercise (133-138 mEq·l-1). The main effect of HS was to expand plasma volume (142 ±33 vs. -58 ±32 vs. -193±37 ml; P<0.05) so that subjects were less hypo hydrated after 240 min (0.19 ±0.27 vs. 1.23 ±0.27 vs. 1.18 ±0.27%; P<0.05 for HS, MS & LS). We conclude that the addition of high (100 mEq·l-1) concentrations of Na+ to a CHO solution ingested during prolonged exercise has a positive effect on fluid balance.