Sweat sodium loss influences serum sodium concentration in a marathon

The aim of this investigation was to determine the influence of sweat electrolyte concentration on body water and electrolyte homeostasis during a marathon. Fifty-one runners completed a marathon race in a warm and dry environment (24.4 ± 3.6 °C). Runners were classified as low-salt sweaters (n = 21; <30 mmol/L of sweat Na⁺ concentration), typical sweaters (n = 20; ≥30 and <60 mmol/L of sweat Na⁺ concentration), and salty sweaters (n = 10; ≥60 mmol/L of sweat Na⁺ concentration). Before and after the race, body mass and a sample of venous blood were obtained. During the race, sweat samples were collected by using sweat patches, and fluid and electrolyte intake were recorded by using self-reported questionnaires. Low-salt, typical and salty sweaters presented similar sweat rates (0.93 ± 0.2, 0.92 ± 0.29, 0.99 ± 0.21 L/h, respectively), body mass changes (-3.0 ± 1.0, -3.3 ± 1.0, -3.2 ± 0.8%), total Na⁺ intake (12.7 ± 8.1, 11.5 ± 9.7, 14.5 ± 16.6 mmol), and fluid intake (1.3 ± 0.8, 1.2 ± 0.8, 1.2 ± 0.6 L) during the race. However, salty sweaters presented lower post-race serum Na⁺ concentration (140.8 ± 1.3 vs 142.5 ± 1.1, 142.4 ± 1.4 mmol/L; P < 0.01) and serum osmolality (297 ± 6 vs 299 ± 5, 301 ± 6 mOsm/kg; P < 0.05) than low-salt and typical sweaters. Sweat electrolyte concentration could influence post-race serum electrolyte concentration in the marathon. However, even the saltiest sweaters did not develop exercise-associated hyponatremia or associated symptoms.

CFTR genotype-related body water and electrolyte balance during a marathon

The aim of this investigation was to determine the influence of CFTR genotype on body water and electrolyte balance during a marathon. Fifty-one experienced runners completed a marathon race. Before and after the race, body mass and a sample of venous blood were obtained. During the race, sweat samples were collected using sweat patches, and fluid and electrolyte intake were obtained using self-reported questionnaires. Thirty-eight participants (74.5% of the total) were 7T/7T homozygotes, 11 (21.6%) were 7T/9T heterozygotes, and one participant presented the rare genotype 5T/7T. Another participant with 9T/9T presented the mutation p.L206W. Participants with 7T/7T showed higher sweat sodium concentrations (42.2 ± 21.6 mmol/L) than 7T/9T (29.0 ± 24.7 mmol/L; P = 0.04). The runner with the 5T/7T genotype (10.2 mmol/L) and the participant with the p.L206W mutation (20.5 mmol/L) exhibited low-range sweat sodium concentrations. However, post-race serum sodium concentration was similar in 7T/7T and 7T/9T (142.1 ± 1.3 and 142.4 ± 1.6 mmol/L, respectively; P = 0.27) and did not show abnormalities in participants with the 5T/7T genotype (140.0 mmol/L) and the p.L206W mutation (143.0 mmol/L). Runners with the CFTR-7T/7T genotype exhibited increased sweat sodium concentrations during a marathon. However, this phenotype was not related with increased likelihood of suffering body water and electrolyte imbalances during real competitions.