The BASES expert statement on interventions for improving performance in the heat

The detrimental influence of heat on prolonged exercise is well established, yet many individuals are required to exercise in the heat. This BASES Expert Statement presents practical, evidence-based interventions, for able-bodied individuals performing continuous or intermittent exercise (>~30 minutes) in hot environments. Acclimation (laboratory)/acclimatisation (natural environment) to heat (HA) reduces initial deep-body temperature and increases heat dissipation rates. HA is achieved by repeatedly elevating deep-body and skin temperature and eliciting pronounced sweating, typically through exercise in a hot environment, although warm-baths following training are effective. 60-90 minute daily exposures are common and ~10 days is superior to ~5 days. Short programmes still elicit significant adaptations although adaptation may be slower in females. Maintaining thermal-strain, by increasing work-rate or environmental stress as the individual adapts, should be superior to exercise at a fixed work-rate and environmental stress, where the stimulus diminishes as the individual adapts. A target deep-body temperature of ~38.5°C is common; higher temperatures appear unnecessary and increase heat-illness risk. HA is well-retained for 7 days and any benefits lost within a month of HA are reacquired with 2-4 further exposures. Individuals should commence exercise in the heat in a euhydrated state and prevent excessive dehydration (>2% body weight loss from water deficit), which impairs heat-loss. However, excessive drinking can cause hyponatremia, which is life-threatening and individuals should drink to thirst unless they have a prescribed fluid-strategy. Pre- or per-(during) exercise cooling techniques can be effective and include: i) external cooling e.g. cool water immersion, cooling garments; ii) internal cooling e.g. ingesting cold/ice-slurry beverage; iii) mixed-methods (multiple internal and/or external methods). Water immersion is often used, with temperatures of 12-18°C, durations of 20-30 minutes, and deep-body temperature monitoring for safety. Typically, 7.5 g·kg^-1 body mass of cold-liquids (carbohydrate/electrolytes) or ice-slurry are ingested in the 20-30 minutes before exercise. Theoretically, mixed-methods should generate the greatest heat-sink. Phase change garments e.g. ‘ice vests’ may inhibit some evaporation and may be more appropriate when evaporative heat-loss is restricted e.g. protective clothing. The efficacy of ventilated vests is reduced with hotter, more humid air. Some interventions induce cool sensations with minimal influence on thermal state e.g. menthol, ice-packs on the neck. We advise caution with perceptual manipulations, which may result in dangerously high body temperatures whilst masking signs and symptoms of heat-illness. The optimal intervention(s) must be determined on a case-by-case basis taking into account mechanisms, efficacy, practical and logistical constraints and safety issues.