Serum Sodium Concentration Changes Are Related to Fluid Balance and Sweat Sodium Loss

PAHNKE, M. D., J. D. TRINITY, J. J. ZACHWIEJA, J. R. STOFAN, W. D. HILLER, and E. F. COYLE. Serum Sodium Concentration Changes Are Related to Fluid Balance and Sweat Sodium Loss. Med. Sci. Sports Exerc., Vol. 42, No. 9, pp. 1669-1674, 2010. 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 (R) triathlon. Methods: Endurance trained athletes (N = 46, age = 24-67 yr) were studied during 30 min of stationary cycling at 70%-75% of HRmax in a warm outdoor laboratory (26.4 degrees C +/- 1.7 degrees C wet bulb globe temperature [WBGT], 28.3 degrees C +/- 1.2 degrees C dry bulb [DB]) 3-7 d before race day. Sweat sodium concentration was measured from absorbent patches on the forearm and scapula, and sweating rate was derived from changes in body mass. Before and after the 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 men, the change in serum sodium concentration during the race was correlated with relative sweating rate (mL.kg(-1).h(-1); r = -0.49, P = 0.012), rate of sweat sodium loss (mEq.kg(-1).h(-1); 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 men (R-2 = 0.46). In women, body mass change alone was significantly correlated with the change in serum sodium concentration (r(2) = 0.31). The rate of sodium intake (mEq.kg(-1).h(-1)) was related to the rate of sweat sodium loss in women (mEq.kg(-1).h(-1); r = 0.64, P = 0.035) but not in men (r = 0.27, P = 0.486). Conclusion: Changes in serum sodium concentration during an ultraendurance triathlon are significantly related to interactions of fluid balance, sweat sodium loss, and sodium ingestion.