Serum sodium concentration changes are related to fluid balance and sweat sodium loss

Journal Title(Medline/Pubmed accepted abbreviation): Med Sci Sports Exerc
Page numbers:1669-74

Summary of Background and Research Design

Background:Large reductions in serum sodium concentration have been observed during prolonged exercise in military personnel and athletes. Serum sodium concentration reflects the sodium concentration in extracellular fluid spaces. Therefore, reduction in sodium concentration can be attributed to either decreased sodium content or increased water volume in extracellular fluid caused by excessive fluid consumption. Although serum sodium reduction may occur via hyperhydration, it may also occur with heavy sweating, when ingested fluid replaces less sodium than that which was lost in sweat.

Hypothesis: The authors hypothesized that changes in serum sodium concentration during ultra-endurance exercise are related to both fluid balance (hydration) and sodium losses attributed to sweat.

Subjects:Healthy male (n = 26) and female (n = 20) athletes participating in the 2003 Hawaii Ironman ® Triathlon World Championship who signed approved consent forms were enrolled in the study. Mean age ± standard deviation (SD) for male and female subjects was 44.5 ± 11.4 years and 41.8 ± 9.9 years, and mean weight ± SD was 77.0 ± 10.3 kg and 61.8 ± 5.6 kg, respectively. In the weeks preceding the study, subjects were required to exercise ? 3 consecutive days at temperatures > 30° C before the study in addition to their normal aerobic training.

Experimental design:Prospective, observational

Treatments and protocol:
Sweating analysis trial
Subjects participated in a sweating analysis trial consisting of an 8-minute warm-up and 30-minute exercise bout on a stationary bicycle. The trial was conducted 3 to 7 days before the race to establish baseline body mass, sweating rate, and serum sodium and sweating sodium concentrations. Waterproof, absorbent sweat patches were applied to the forearm and scapula of subjects and they received 400 mL of a 6% carbohydrate-electrolyte beverage to consume during the 30-minute exercise bout. Serum and sweat sodium concentrations were determined by chemical analysis of blood samples and sweat patches, respectively, and sweating rate was calculated from change in body mass after the 30-minute exercise bout.
Race day study
On the day of the race, baseline body mass and blood samples were obtained from a subset of subjects (n = 20) within 2 hours of starting and from all subjects (N = 46) within 5 to 15 minutes of finishing the race and were used to calculate serum and sweat sodium concentrations. To calculate sodium intake during the race, self-reported food and fluid intake were obtained from a subset of subjects (n = 20) at the 56-mile mark, at completion of the bike segment, and/or at the end of the race. Total sodium ingestion was calculated by adding sodium consumed from pills, food, and fluids. Food, fluid, and sodium supplementation were not restricted nor supplied by investigators during the race. Mean, SD, and coefficient of variation were calculated for descriptive variables.

Summary of Research Findings
  • In the sweating analysis trial, males had somewhat higher sweat sodium losses (27%) than females, although this was not statistically significant ( P = .155)
  • Overall, there was a significant negative correlation between body mass change and serum sodium level ( r = –0.55, P = .001), such that a 1-kg body mass loss was associated with a 1.7-mEq/L drop in serum sodium concentration.
  • Rate of sweat sodium loss and body mass change together accounted for 46% of the change in serum sodium concentration in male subjects ( R 2 = 0.46)
  • Body mass change alone accounted for 31% of the change in serum sodium concentration in female subjects ( r 2 = 0.31; P = .010)
  • Based on nutritional intake during the race, the rate of sodium intake was related to the rate of sweat sodium loss in female subjects ( r = 0.64; P = .035) but not in male subjects ( r = 0.27; P = .486)
  • Despite the gender differences noted above, males and females had similar postrace declines in serum sodium levels (–3.4 ± 4.6 vs. –1.9 ± 4.9 mEq/L, respectively, P = .284)

Interpretation of findings/Key practice applications:

Specifically, changes in serum sodium concentration during an ultra-endurance triathlon were attributed to changes in body mass and rate of sweat sodium loss in male subjects, and in changes in body mass in female subjects. Compared with males, females more closely matched their sweat sodium losses with sodium intake, which was more easily accomplished because they had lower rates of sweat sodium losses than male subjects. Limitations of the study include: 1) potential sources of error because the study was conducted in a non-laboratory setting; 2) sweat sodium concentration may have been overestimated if the sweat patches used in the trial became oversaturated; 3) nutritional intake was not standardized during the race; and 4) nutritional intake after the race was based on self-reporting.
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