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

Abstract

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_gi)) temperature was measured continuously and dehydration determined using body mass changes.

Results: Except for 60% V̇O_2peak in WARM, in which adults exhibited a greater T_gi rise compared with 10-13 years, there was no effect of age on T_gi 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.

Keywords: Children; Dehydration; Exercise; Sweat.

Figure 1. The change in core temperature during 45 min of exercise in children and adults in WARM conditions (30°C, 40% RH) and the association between age and ∆T_gi for all participants aged <17 years. Individual data for boys/men (blue) and girls/women (yellow) are plotted. Some individual data points may not be visible due to data overlap. MET, metabolic equivalent; RH, relative humidity.

Figure 2. The change in core temperature during 45 min of exercise in children and adults in HOT conditions (40°C, 30% RH) and the association between age and ∆T_gi for all participants aged <17 years. Individual data for boys/men (blue) and girls/women (yellow) are plotted. Some individual data points may not be visible due to data overlap. MET, metabolic equivalent; RH, relative humidity.

Figure 3. Whole-body sweat rate and total body water fluid deficit (%) during exercise in WARM conditions (30°C, 40% RH). Individual data for boys/men (blue) and girls/women (yellow) are plotted. Some individual data points may not be visible due to data overlap. MET, metabolic equivalent; RH, relative humidity; TBW, total body water; WBSR, whole-body sweat rate.

Figure 4. Whole-body sweat rate and total body water fluid deficit (%) during exercise in HOT conditions (40°C, 30% RH). Individual data for boys/men (blue) and girls/women (yellow) are plotted. Some individual data points may not be visible due to data overlap. MET, metabolic equivalent; RH, relative humidity; TBW, total body water; WBSR, whole-body sweat rate.

Figure 5. Predicted versus measured whole-body sweat rate with the dotted line indicating the line of identity (y=x). Bland-Altman plots illustrating mean bias (dotted line) and 95% CIs (black solid lines) expressed as a % of total body water loss calculated using separate equations for boys and girls. Red shaded area indicates a priori acceptable limits of agreement for predicting sweat rate over a 90 min exercise period (±2% of total body water²⁹). Individual data for boys (blue) and girls (yellow) are plotted. TBW, total body water.