Higher sweat rate values have been reported during intermittent compared to continuous type exercise in hot conditions in the laboratory. Studies in the training field are scarce. Objective: Document sweat rate, rehydration and core temperature (Tc) during training in long duration-continuous (CON) and intermittent (INT) type sports, and determine the presence of a mutation in the CFTR gene in athletes with high sodium concentration ([Na+]) in sweat. Methods: Athletes (CON =50 and INT=123) were tested during training in tropical climate. Sweat rate, sweat [Na+], Tc, de­hydration level, and presence of ? F508 gene mutation in the CFTR gene in athletes with high sweat [Na+] were evaluated. Results: Sweat rate was higher in CON (1.5 ± 0.4 L/h) compared to INT (1.2 ± 0.5 L/h) and CON athletes finished training with higher dehydration (2.1 ± 0.8 vs 1.2 ± 0.7%) P < 0.05. Sweat [Na+] was higher in INT (62.0 ± 21.1 mmol/L) compared to CON (53.9 ± 18.1 mmol/L), P < 0.05. A tennis player with high [Na+] was heterozygous for the ?F508 mutation. Average and highest Tc was similar for CON (38.4 ± 0.3 and 38.8 ± 0.4ºC) and INT (38.3 ± 0.3 and 38.7 ± 0.4 ºC), P > 0.05. Conclusion: During training in a tropical climate, sweat loss and dehydration level are lower, and fluid intake is higher in intermittent compared to continuous type sports. Core temperature may rise to a similar level in intermittent type sports due to the repeated high intensity bouts and/or the effects of clothing worn while training in hot venues. Healthy athletes with high [Na+] in sweat who are heterozygous carriers of CFTR mutations may be at increased risk for hyponatremic dehydration and whole-body muscle cramps.
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