Cross-Adaptation: Heat and Cold Adaptation to Improve Physiological and Cellular Responses to Hypoxia

Abstract

To prepare for extremes of heat, cold or low partial pressures of oxygen (O2), humans can undertake a period of acclimation or acclimatization to induce environment-specific adaptations, e.g. heat acclimation (HA), cold acclimation (CA), or altitude training. While these strategies are effective, they are not always feasible due to logistical impracticalities. Cross-adaptation is a term used to describe the phenomenon whereby alternative environmental interventions, e.g. HA or CA, may be a beneficial alternative to altitude interventions, providing physiological stress and inducing adaptations observable at altitude. HA can attenuate physiological strain at rest and during moderate-intensity exercise at altitude via adaptations allied to improved O2 delivery to metabolically active tissue, likely following increases in plasma volume and reductions in body temperature. CA appears to improve physiological responses to altitude by attenuating the autonomic response to altitude. While no cross-acclimation-derived exercise performance/capacity data have been measured following CA, post-HA improvements in performance underpinned by aerobic metabolism, and therefore dependent on O2 delivery at altitude, are likely. At a cellular level, heat shock protein responses to altitude are attenuated by prior HA, suggesting that an attenuation of the cellular stress response and therefore a reduced disruption to homeostasis at altitude has occurred. This process is known as cross-tolerance. The effects of CA on markers of cross-tolerance is an area requiring further investigation. Because much of the evidence relating to cross-adaptation to altitude has examined the benefits at moderate to high altitudes, future research examining responses at lower altitudes should be conducted, given that these environments are more frequently visited by athletes and workers. Mechanistic work to identify the specific physiological and cellular pathways responsible for cross-adaptation between heat and altitude, and between cold and altitude, is warranted, as is exploration of benefits across different populations and physical activity profiles.

Heat acclimation does not affect maximal aerobic power in thermoneutral normoxic or hypoxic conditions

“…In contrast, an acute plasma volume expansion does not provide any benefit to CO max or in HYP (Calbet et al., ). Thus, HA‐enhanced oxygen delivery has been suggested to benefit submaximal rather than maximal exercise in HYP (Gibson, Taylor, Watt, & Maxwell, ). As a result, in HYP is reduced in the same proportion that arterial oxygen content is reduced (Mollard et al., ).…”

Once- and twice-daily heat acclimation confer similar heat adaptations, inflammatory responses and exercise tolerance improvements

“…), and potential benefits across environmental stressors (Gibson et al. , ). We also highlight a need to investigate the precise effect of consecutive and nonconsecutive TDHA in females, who experience different thermoregulatory adaptation kinetics to males (Mee et al.…”

“…Collecting additional biomarker measures and across more time-points during the recovery phase (e.g., 1-24 h) would provide further insight into the inflammatory responses and potential maladaptive influences on the magnitude and kinetics of heat adaptation. An extension of this work would also examine intracellular heat shock proteins (Kuennen et al 2011) and the relevant gene transcripts (Gibson et al 2015a) to elucidate the impact of TDHA versus ODHA on attaining thermotolerance (Kuennen et al 2011), and potential benefits across environmental stressors (Gibson et al 2015c(Gibson et al , 2017a. We also highlight a need to investigate the precise effect of consecutive and nonconsecutive TDHA in females, who experience different thermoregulatory adaptation kinetics to males (Mee et al 2015).…”

Application of evidence-based recommendations for heat acclimation: Individual and team sport perspectives

“…Regardless, no evidence indicates HA impairs aerobic performance in cooler conditions, so HA integrated into annual training plans may improve performance in both hot and cooler conditions without compromising higher priority training objectives. HA may also lessen physiological strain at altitude [27,28] through epigenetic [29], cellular [30], and system level adjustments [31]. The potential of improved performance via cross-tolerance among environmental conditions is an exciting new area of discovery.…”