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, V˙
O2max 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% VO2max, 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.