Abstract

Background: Numerous laboratory based studies have documented that aggressive hydration strategies (∼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 T_core 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)°C (range 19–26°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)°C. Body mass significantly declined during the race by 2.3 (1.2) kg (−3.0 (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  =  −0.37).

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.