Core temperature and hydration status during an Ironman triathlon - Commentary

Background: Numerous laboratory based studies have documented that aggressive hydration strategies (similar to 1-2 litres/h) are required to minimise a rise in core temperature and minimise the deleterious effects of hyperthermia on performance. However, field data on the relations between hydration level, core body temperature, and performance are rare. Objective: To measure core temperature (T-core) in triathletes during a 226 km Ironman triathlon, and to compare Tcore with markers of hydration status after the event. Method: Before and immediately after the 2004 Ironman Western Australia event (mean (SD) ambient temperature 23.3 (1.9) degrees C (range 19-26 degrees C) and 60 (14)% relative humidity (44-87%)) body mass, plasma concentrations of sodium ([Na+]), potassium ([K+]), and chloride ([Cl-]), and urine specific gravity were measured in 10 well trained triathletes. T-core was measured intermittently during the event using an ingestible pill telemetry system, and heart rate was measured throughout. Results: Mean (SD) performance time in the Ironman triathlon was 611 (49) minutes; heart rate was 143 (9) beats/min (83 (6)% of maximum) and T-core was 38.1 (0.3) degrees C. Body mass significantly declined during the race by 2.3 (1.2) kg (23.0 (1.5)%; p < 0.05), whereas urine specific gravity significantly increased (1.011 (0.005) to 1.0170 (0.008) g/ml; p < 0.05) and plasma [Na+], [K+], and [Cl-] did not change. Changes in body mass were not related to finishing T-core (r = -0.16), plasma [Na+] (r = 0.31), or urine specific gravity (r = 20.37). Conclusion: In contrast with previous laboratory based studies examining the influence of hypohydration on performance, a body mass loss of up to 3% was found to be tolerated by well trained triathletes during an Ironman competition in warm conditions without any evidence of thermoregulatory failure.