Thermoregulation in the heat: Not so black and white

There is a scarcity of research literature that includes people of colour. Blacks and Latinos make up 30% of the US population but account for only 6% of all participants in federally funded clinical trials (Oh et al., 2015). Lack of representation in these studies might explain how treatment interventions are more efficacious in a white population. Furthermore, this disparity might contribute to the increased prevalence of cardiovascular disease, type II diabetes and cancer in people of colour. Thus, our knowledge of differences in physiological mechanisms, including thermoregulatory function, across race and ethnicity is lacking. It has long been recognized that indigenous populations of hot climates have greater heat tolerance than non-natives. Intrinsic to this discussion is the extent to which increased heat tolerance in various races and ethnicities is genetic in origin or involves environmental factors (e.g. heat acclimatization occurring in residents of hotter climates). Using a unique experimental design that controlled for environmental factors, Muia, Notley, Saci, D'Souza, and Kenny (2020) provide in this issue of Experimental Physiology compelling research findings comparing Canadian residents who were second-generation or higher black-African men (i.e. equatorial descent) with white men (i.e. ancestrally European). Specifically, these groups were compared with respect to variables of heat exchange measured by direct calorimetry during various intensities of exercise performed in hot, dry conditions. The authors were careful to ensure that both groups were similar with respect to age, body surface area, body fat, aerobic capacity and physical activity. Across all exercise bouts ranging from light to vigorous, there were no differences between black and white groups with respect to core and skin temperature, dry and evaporative heat loss and metabolic heat production. Collectively, these findings convincingly indicate that blacks and whites do not differ genotypically in their thermoregulatory response to exercising in the heat. Thus, any apparent racial differences in heat tolerance appear to be attributable to environmental adaptations (i.e. phenotypic adaptations) that occur during a lifetime. Humans are tropical animals that can tolerate and adapt remarkably well to heat. In 1775, Sir Charles Blagden and his co-investigators performed a series of experiments where they (including his dog) were unharmed after enduring a hot ∼100–125°C room for 15–20 min. He reasoned that humans could withstand extreme heat as long as sweat could be generated continuously and evaporated from the skin. In the 1940s, Sid Robinson developed the ‘traditional protocol’ for heat acclimatization that was evidenced by four physiological adaptations: reduced core temperature, reduced skin temperature, reduced heart rate and increased sweating for a given level of heat exposure (Schneider, 2016). Although these adaptations collectively reduce thermal and cardiac strain, increased sweating is energetically wasteful in the tropics. In a humid environment, increased sweat production may elicit only a negligible to modest increase in evaporative heat loss, thereby leading to more unevaporated sweat (Taylor, 2014). Thus, longer-term heat acclimatization effects in these conditions may include reduced skin blood flow and increased skin temperature as a means of reducing the skin–environment temperature gradient to minimize heat gain. The study by Muia et al. (2020) is unique in that these heat acclimatization effects, which may have influenced other studies that compared across region, would not interfere with their examination of genotypic differences in heat exchange between black and white populations. Based upon their findings and previous studies, the more favourable heat exchange and tolerance to hot, humid conditions occurs as a result of region-specific environmental adaptations and not because of race or ethnicity. The assumption that black individuals are genetically predisposed to withstand heat is a dangerous one. In the USA, the heat-related death rate is approximately four times higher in a black as opposed to a white population. Several factors increase the risk of heat-related death, including greater frequency and severity of heatwaves and less access to air-conditioning and equitable health care. These risks are further compounded by age and the increased prevalence of cardiovascular disease (Kenney, Craighead, & Alexander, 2014). Collectively, future research designs should consider these factors when examining differences in race and ethnicity. In this context, the authors take an important step in clarifying the differences that do, or do not, exist between black and white populations. None declared.

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