A randomized, cross-over trial assessing effects of beverage sodium concentration on plasma sodium concentration and plasma volume during prolonged exercise in the heat

PURPOSE: This study assessed whether increasing sodium in a sports drink above that typical (~ 20 mmol L(−1)) affects plasma sodium and volume responses during prolonged exercise in the heat. METHODS: Endurance trained males (N = 11, 36 ± 14 y, 75.36 ± 5.30 kg, [Formula: see text] O(2max) 60 ± 3 mL min(−1) kg(−1)) fulfilled requirements of the study including one 1-h exercise pre-trial, to estimate fluid losses (to prescribe fluid intake), and two, experimental trials (3-h or until tolerance), in random order, cycling (55% [Formula: see text] O(2max), 34 °C, 65% RH). Beverages contained 6% carbohydrate and either 21 mmol L(−1) (Low Na(+)) or 60 mmol L(−1) sodium (High Na(+)). Analyses included linear mixed models and t-tests. RESULTS: Cycling time was similar 176 ± 9 min (Low Na(+)); 176 ± 7 min (High Na(+)). Fluid intake was 1.12 ± 0.19 L h(−1); 1.14 ± 0.21 L h(−1), resp. Body mass change was − 0.53 ± 0.40%; − 0.30 ± 0.45%, resp. Sodium intake was 69 ± 12 mmol; 201 ± 40 mmol, resp. Plasma sodium concentration was greater in High Na(+) than Low Na(+) (p < 0.001); decreasing in Low Na(+) (− 1.5 ± 2.2 mmol L(−1)), increasing in High Na(+) (0.8 ± 2.4 mmol L(−1)) (p = 0.048, 95% CI [− 4.52, − 0.02], d = 0.99). Plasma volume decreased in Low Na(+) (− 2 ± 2%) but remained unchanged in High Na(+) (0 ± 3%) (p = 0.01, 95% CI [− 3.2, − 0.5], d = 0.80). CONCLUSIONS: When conducting prolonged exercise in the heat, those who fully hydrate would benefit by increased sodium content of the beverage by improved plasma volume and sodium maintenance. Australian New Zealand Clinical Trials Registry (ACTRN12616000239460) 22/02/16. SUPPLEMENTARY INFORMATION: The online version contains supplementary material available at 10.1007/s00421-022-05025-y.