n International SportMed Journal - Exercise in the heat : old ideas, new dogmas : review article




A remarkable feature of the human species is our great capacity to lose heat and to regulate our body temperatures when exposed to heat. The Heinrich (Hunting) hypothesis theorises that this capacity evolved in early hominids and provided an extraordinary evolutionary advantage since it allowed our human ancestors to chase nutritionally-dense but non-sweating mammals like large antelope, to their (lesser) limits of thermoregulatory failure. In this fatigued state, the exhausted animals were more easily killed. It is further hypothesised that it was the consumption of the high protein / fat diet made possible by the capture of such animals that fuelled the development of the higher brain centres on which superior human intelligence depends. But the key physiological point is that during the hunt, early hominids had no access to fluid, the carrying of which would have hindered their ability to run. Thus for their evolutionary progress to have occurred, humans had to evolve a sweating mechanism to prevent overheating during exercise, as well as the capacity to continue exercising in the heat even though they were becoming increasingly dehydrated as the duration of the hunt increased and the environmental conditions likely became more severe. <br>Until 1969 the advice given to endurance athletes mirrored this understanding of our evolutionary biology. Thus human athletes were advised &lt;I&gt;not&lt;/I&gt; to drink during exercise. But the development of the world's first "sports drink" in Florida in the 1960s led gradually, but perhaps inevitably, to the promotion of a novel dogma that humans need to drink "as much as tolerable" during exercise if they are to avoid heatstroke and to optimise their performances. <br>In this article this and three other dogmas that have evolved simultaneously over the past 35 years are analysed. These are: (i) that dehydration is the most important determinant of the body temperature during exercise; (ii) that athletes collapse after exercise because of a circulatory collapse caused by dehydration and hyperthermia; and (iii) that heatstroke always occurs in otherwise normal, healthy athletes who simply exercise too hard for too long in exceptionally hot conditions. <br>Rather, it is argued in this paper that (i) all the published evidence supports the belief that health and performance during exercise are optimised by drinking according to the dictates of thirst ("ad libitum"); (ii) the brain sets both the work rate and the rectal temperature during exercise specifically to insure that heatstroke will almost never occur in otherwise healthy humans. As a result, the rectal temperature is determined principally by the exercising work rate; (iii) that post-exercise collapse in athletes who remain conscious is due to a low peripheral vascular resistance to which dehydration makes essentially no contribution; and (iv) that heatstroke is more likely due to an exaggerated and explosive thermogenesis that develops in genetically-predisposed individuals on exposure to exercise and other triggering environmental factors. It is also suggested that the skeletal muscles are the site of this florid thermogenesis and that the leakage of toxic proteins into the circulation from damaged muscles, and not the hyperthermia, is the more likely cause of the multiple organ failure that typifies fatal heatstroke.


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