PURPOSE: This study determined if changes in serum sodium concentration are related to fluid balance as well as sweat sodium losses in triathletes competing in the Hawaii Ironman triathlon.
METHODS: Endurance trained athletes (n=46, 24-67y) were studied during 30 minutes of stationary cycling at 70-75% of maximum heart rate, in a warm outdoor laboratory (26.4+/-1.7 degrees C WBGT, 28.3+/-1.2 degrees C db), 3-7d prior to race day. Sweat sodium concentration was measured from absorbent patches on the forearm and scapula and sweating rate derived from changes in body mass. Pre- and post-race serum sodium concentration, body mass, and nutritional intake during the race were also measured (n=46). Sweating and race day comparisons and changes in serum sodium concentration were analyzed via student's t-test, correlation, and multiple regression.
RESULTS: In males, the change in serum sodium concentration during the race was correlated with relative sweating rate (ml/kg/h; r=-0.49, p=0.012), rate of sweat sodium loss (mEq/kg/h; r=-0.44, p=0.023), and body mass change (kg; r=-0.54, p=0.005). Together, the rate of sweat sodium loss and body mass change accounted for 46% of the change in serum sodium concentration in males (R=0.46). In females, body mass change alone was significantly correlated with the change in serum sodium concentration (r=0.31). The rate of sodium intake (mEq/kg/h) was related to the rate of sweat sodium loss in females (mEq/kg/h; r=0.64, p=0.035), but not males (r=0.27, p=0.486).
CONCLUSION: Changes in serum sodium concentration during an ultra-endurance triathlon are significantly related to interactions of fluid balance, sweat sodium loss, and sodium ingestion.
Med Sci Sports Exerc. 42(9):1669-1674.