We assessed the fluid balance, whole body sweat rate (WBSR) and sweat sodium concentration ([Na+]sweat) of English Premier League (EPL) soccer players during pre-season training and used predictive modelling to determine theoretical changes in plasma sodium concentration ([Na+]plasma). Players (25 ± 4 years, 1.83 ± 0.06 m, 81.1 ± 7.4 kg) were monitored during two training sessions (n = 14, Session 1; n = 17, Session 2) in temperate climates (19-23°C, 45-67% relative humidity). Training load was monitored via global positioning system, sweat loss was assessed from changes in body mass (BM) after correcting for fluid intake, and [Na+]sweat was measured using absorbent patches. Mean BM loss was 1.08 ± 0.72% and 1.28 ± 0.65%, with sweat losses of 1.67 ± 0.49 L and 1.88 ± 0.59 L in Sessions 1 and 2, respectively. Mean [Na+]sweat was 51.3 ± 14.6 mmol·L-1 (Session 1) and 39.9 ± 15.3 mmol·L-1 (Session 2), with total sodium losses of 1929 ± 771 mg and 1730 ± 813 mg, respectively. Predictive modelling indicated 4/14 (~29%) players in Session 1, and 1/17 (~6%) players in Session 2, would have mildly reduced [Na+]plasma following training (all <2 mmol·L-1), due to either near-complete fluid replacement or high [Na+]sweat. Although substantial interindividual variations were observed in fluid and sodium balance among players, modelling indicates that Na+ supplementation is unnecessary for soccer players during training in a temperate climate. Measuring [Na+]sweat is therefore unlikely to inform practical recommendations under these conditions.
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