Sodium Balance And Sweat Loss During The Hawaii IRONMAN Triathlon

Abstract

PURPOSE To determine the variability in sweating rate and sweat sodium concentration (sweat-Na) and its relationship to changes in serum sodium concentration (serum-Na) in ultra-endurance triathletes competing in the 2003 Ironman Triathlon World Championship. METHODS During the sweat analysis trial, seventy-one heat-acclimatized males and females (22–74y) exercised on a stationary bicycle at 70–75% of HRmax for 30 minutes following an 8 minute warm-up. All trials took place in warm conditions (26.4 ±0.2°CWBGT, 28.5 ± 0.2°C dry bulb, 66.0 ± 0.0% RH) in an outdoor field laboratory in Kailua-Kona, HI, 3–7d prior to the race. Serum-Na and skin-fold thickness were measured prior to exercise. Sweating rate, determined via change in body mass, and sweat-Na were measured during the 30 min exercise bout. Forty-six male and female subjects who participated in the sweating analysis trial were also studied on race-day. Pre- and post-race serum-Na and body mass were measured and race nutritional intake was also recorded. RESULTS The subjects exhibited a large amount of variability in absolute and relative sweating rate (1.39±0.05 L/hr, coefficient of variation (CV) = 30.8%; 19.4 ± 0.7 ml/kg/hr, CV = 28.5%), sweat-Na (43.4±2.1 mEq/L, CV = 40.0%), and absolute and relative sweat-Na loss(61.6±4.1 mEq/hr, CV = 56.0; 0.87 ± 0.06 mEq/kg/hr, CV = 54.2%). In males, the decline in race-day serum-Na was correlated with relative sweat rate, relative rate of sweat-Na loss, and body mass change (r=−0.413, p<0.05; r=−0.457, p<0.05; r=−0.549, p<0.01). Together, the rate of relative sweat-Na loss and body mass change were highly related to changes in serum-Na (r=0.686, p<0.05). In females, body mass changes alone were highly associated with declines in serum-Na (r=0.614, p<0.05). Females had a significant relationship between rate of absolute sweat-Na loss and Na intake (r=0.637, p<0.05), while males did not (r=0.280, p=0.466). CONCLUSIONS Changes in serum-Na during an ultra-endurance triathlon were related to the interactions of sweat-Na loss, sodium ingestion, and fluid balance. Supported by the Gatorade Sports Science Institute.