Thermoregulation and dehydration in children and youth exercising in extreme heat compared with adults.

Objective: To compare hyperthermia and physiological dehydration risk during exercise heat stress between children of different ages and adults and evaluate an existing adult sweat rate calculator in children.

Methods: 68 fit and recreationally active children aged 10-16 years (31 girls), and 24 adults aged 18-40 years (11 females) completed three separate 45 min treadmill walking/running trials at different intensities on different days at 30°C, 40% relative humidity (RH) (WARM) or 40°C, 30% RH (HOT). Exposures were randomised to elicit intensities scaled to (1) fitness, (2) mass and (3) surface area. Core (gastrointestinal (T)) temperature was measured continuously and dehydration determined using body mass changes.

Results: Except for 60% V̇O in WARM, in which adults exhibited a greater T rise compared with 10-13 years, there was no effect of age on T during exercise (p≥0.176). Physiological rates of dehydration were not affected by age in WARM (p≥0.08) or HOT (p≥0.08). Mean predicted sweat rate error was +0.08 kg/hour (95% CIs: -0.10, +0.25) across WARM and HOT, and 80.5% of variability in sweating was explained by the adult sweat rate calculator.

Conclusions: Using the most comprehensive paediatric exercise heat stress dataset from a single study to date, we show that children aged 10-16 years are at a similar risk of hyperthermia and dehydration as adults during exercise up to 40°C. This supports recent changes to paediatric sport heat policies that were based on limited data. Practitioners can potentially reduce behavioural dehydration risks from inadequate fluid consumption using an existing adult sweat rate calculator for children.