Trained Subjects Excrete More Sodium in Sweat than Untrained when Normalized by Sweat Rate

Increases in sweat rate with exercise intensity are paralleled by augmented sweat sodium concentration ([Na+]sweat)1. Sweat rate is higher in aerobically trained than in untrained subjects when they exercise at the same relative intensity2. However, it is unclear how training affects [Na+]sweat when the difference in sweat rate is taken into account. PURPOSE: To determine the differences in [Na+]sweat of aerobically trained vs. untrained subjects when exercising at different intensities. METHODS: Ten endurance-trained (VO2peak = 60 ± 6 mL · kg-1 · min-1) and ten untrained (VO2peak = 44 ± 3 mL · kg-1 · min-1) men pedalled in a hot-dry environment (36 °C; 23 % rh; 2.5 m · s-1 wind speed) at three intensities (40, 60, and 80% of VO2peak). Sweat was collected with patches positioned on the lower back. Sweat rate was assessed by differences in body weight and [Na+]sweat analyzed by flame photometry. Student's t-test was used to detect differences between groups. RESULTS: (Figure 1; y-intercept of Trained higher than Untrained; P < 0.05).FigureCONCLUSIONS: Coinciding with previous data, exercise intensity increased sweat rate and [Na+]sweat in both groups. The novel finding is that training shifted the [Na+]sweatvs. sweat rate relationship to the left; thus increasing the excretion of sodium in sweat for a given sweat rate around 23 mEq · L-1. The slopes of the relationship were not significantly changed between groups but there were a significant increase in the mean y-intercept for the trained. The reasons why aerobically trained subjects conserved less sodium (i.e., excrete more in sweat) during exercise in the heat in a wide range of exercise intensities is not evident in the present data. 1. Buono et al., JAP, 2007 2. Fritzche et al., JAP, 2000