Core temperature and hydration status during an Ironman triathlon

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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 (Tcore) in triathletes during a 226 km Ironman triathlon, and to compare Tcorewith 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. Tcorewas 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 Tcorewas 38.1 (0.3)°C. Body mass significantly declined during the race by 2.3 (1.2) kg (−3.0 (1.5)%; p+], [K+], and [Cl−] did not change. Changes in body mass were not related to finishing Tcore(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.

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