.... Allied Forces are currently involved in theatres of operation that require the donning of protective clothing such as body armour as well as exposure to ambient temperatures that can reach or exceed 50ºC. These conditions together with the requirements to maintain moderate to heavy work efforts for prolonged periods of time can create uncompensable heat stress where body temperature and heat storage continue to rise to dangerously high levels. The data presented here clearly reveal the advantage and importance of maintaining a high level of aerobic fitness for sustaining one’s operational effectiveness for longer periods of time as core temperature increases. Thus the aerobically fit soldier will be able to march further, fight longer, and make better decisions than the unfit soldier in these extreme environments. However, our findings raise as many questions as they appear to answer. For example, will the endurance trained soldier be at risk for exertional heat injury sooner if fluid replacement can not match their elevated rates of fluid loss? Some of our additional data analyses, which are not presented here, indicate that this may be the case. Is there a threshold level of aerobic fitness associated with enhanced thermotolerance or is some regular physical activity, regardless of intensity, beneficial? Our theory regarding the importance of self-immunization with a small amount of endotoxin leakage with regular exercise implies that a certain redistribution of blood flow away from the gut is required before this leakage occurs. Our data suggest that heart rates around 75-80% of peak values must be attained before endotoxin leakage is present. In reality, it is likely that leakage occurs at somewhat lower heart rates but nevertheless these findings imply that low to moderate levels of exercise in a normothermic environment would not be sufficient to promote this self-immunization. Could the benefits of this selfimmunization occur with repeated exposure to passive heating, albeit probably to higher core temperatures, and could this serve as an alternate strategy for adaptation? Perhaps the best adaptive strategy would be repeated exercise exposure in a hot environment, a process commonly referred to as heat acclimation. Those that are less aerobically fit would probably demonstrate the greatest benefits and cellular adaptations with this approach since the endurance trained individual is already considered partially heat acclimated [15]. We are currently examining these possibilities ....
