Assessing passive heat acclimation as a dual stimulus for erythropoiesis and central cardiovascular adaptation in well‐trained runners

Altitude training has traditionally been utilized by endurance athletes to increase haemoglobin mass (Hbmass) via hypoxia-induced erythropoiesis, enhancing blood oxygen-carrying capacity, which can translate to improved endurance performance at sea level. However, logistical challenges and the feasibility of altitude training prompted interest in developing alternative strategies to elicit similar haematological benefits. Long-term heat acclimation (HA; ≥14 days), typically involving repeated exercise bouts in simulated hot environments (≥35°C), is proposed as a potential substitute for altitude training. Completing HA can increase Hbmass and improve maximal oxygen uptake ( V ̇ O 2 max V_O₂max ), with several protocols demonstrating beneficial erythropoietic responses and corresponding performance benefits. Despite promising findings, HA studies conducted in athlete populations typically implement exercise-induced heat stress, often requiring reductions in training intensity to facilitate protocol completion. This complicates integration with normal training periodization and may reduce feasibility for athletes seeking to maintain high-quality sessions. However, passive heat exposure [e.g. hot-water immersion (HWI) or sauna bathing] may offer a more practical method for achieving heat-induced adaptations without compromising training quality. For example, moderately fit participants conducted 6 weeks of HWI, which subsequently increased V ̇ O 2 max V_O₂max in both normoxic and hypoxic conditions (Rodrigues et al., 2025). However, in normoxia, both control and intervention groups demonstrated comparable V ̇ O 2 max V_O₂max improvements, probably resulting from training novelty (post-exercise HWI) rather than a heat stress-induced effect. Importantly, Hbmass was not measured in the aforementioned study, leaving haematological and cardiac mechanisms underlying performance improvements unclear. A notable gap in the literature is whether the inclusion of passive HA alongside normal training routines induces meaningful haematological adaptations, particularly increases in Hbmass and red blood cell volume (RBCV), over an extended period. Additionally, it is uncertain whether passive heat exposures elicit structural cardiovascular adaptations. While HA expands plasma volume (PV) and may function as a dual stimulus by enhancing cardiac preload, the impact of extended heat exposures on cardiac structure and function is not well understood. Establishing the distinct contributions of haematological and cardiac adaptations is essential as oxygen delivery during maximal exercise depends on both arterial oxygen content and cardiac output. Although Hbmass is a well-established determinant of V ̇ O 2 max V_O₂max , cardiac output driven by stroke volume (SV) and left-ventricular end-diastolic volume (LVEDV) is regarded as the primary limiting factor in trained individuals. In a recent issue of The Journal of Physiology, Jenkins et al. (2025) studied the effect of long-term passive HA on haematological and cardiac adaptations in endurance runners, and the translation to improvements in V ̇ O 2 max V_O₂max . The study of Jenkins et al. (2025) adopted a within-subject, counterbalanced crossover design comparing the physiological and performance effects of 5 weeks of HWI vs. a time-matched control (CON) in endurance-trained runners. Ten well-trained, heat-unacclimated runners ( V ̇ O 2 max V_O₂max = 64.5 ± 8.1 mL kg−1 min−1) completed pre-intervention testing, involving measures of haematological and cardiac function, followed by either chest to shoulder immersion (5 × 45 min week−1; ∼40–42°C) or CON while maintaining habitual training. Following a minimum 5 week washout period, participants completed the alternative condition. Haematological adaptations were assessed weekly using duplicate carbon monoxide rebreathing tests to quantify Hbmass, PV, RBCV and total blood volume. Cardiac function was evaluated before and after intervention using 4D transthoracic echocardiography to determine left ventricular volumes, SV, ejection fraction, myocardial strain and diastolic filling metrics. Exercise performance was assessed through a submaximal lactate-threshold test and a graded maximal treadmill running test to determine V ̇ O 2 max V_O₂max and running economy. Additionally, daily training load and subjective well-being were monitored to ensure changes in outcome variables were not a result of training load alterations between conditions. Statistical analyses were conducted using two-way repeated-measures ANOVAs (or mixed models where appropriate) to assess main effects of time, condition and their interaction. Furthermore, best subset linear mixed-effects regression was used to identify haematological and cardiac predictors of V ̇ O 2 max V_O₂max , with model selection based on Akaike's Information Criterion. The 5 week HWI protocol progressively induced thermoregulatory, cardiovascular and haematological adaptations. Participants exhibited reductions in tympanic temperature from before to after immersion (P = 0.04) and a progressive increase in sweat rate throughout the intervention (P < 0.001), indicating improved heat dissipation. It is important to highlight that positional shifts during immersion may have artificially elevated temperature readings, due to ear canal exposure to high ambient heat and steam. Nevertheless, HWI resulted in a rapid expansion of PV, with an increase of ∼250 mL following the first week. PV remained elevated through week 2 before gradually returning toward pre-intervention levels. This early PV expansion caused transient haemodilution, evidenced by a 1.7% decrease in haematocrit (Hct), and probably served as the initiating signal for erythropoiesis by reducing arterial oxygen content and renal oxygen tension. Correspondingly, RBCV increased and Hbmass increased by 33 g within 4 weeks, consistent with the Critmeter hypothesis, which proposes that Hct is maintained near an optimal set point to balance oxygen delivery and the viscosity associated with higher RBC concentration. The combined increases in PV and RBCV produced a sustained elevation in total blood volume, amounting to an average increase of 284 mL by the end of the intervention. The expansion in blood volume increased cardiac preload, contributing to central cardiovascular remodelling. Structurally, participants demonstrated a 10 mL increase in LVEDV, indicating enhanced ventricular chamber filling. Subsequently, the greater LVEDV most likely contributed to the 7 mL rise in SV and 0.6 L min−1 increase in cardiac output. These observed cardiovascular adaptations probably underpinned the ∼2.7 mL kg−1 min−1 increase in V ̇ O 2 max V_O₂max and accompanied 0.8 km h−1 improvement in speed at V ̇ O 2 max V_O₂max . Regression analysis further identified Hbmass as the strongest predictor of absolute V ̇ O 2 max V_O₂max , underscoring haematological adaptations as an important mechanism driving performance improvements following HWI. The findings of Jenkins et al. (2025) offer several practical insights and highlight important avenues for future passive heating research. The demonstrated efficacy of long-term passive HA is highly relevant for elite athletes, who continuously seek performance-enhancing strategies that do not interfere with existing training practices. By integrating HWI into athletes’ regular training schedules, this strategy allows for the maintenance of normal training loads whilst enhancing both performance and heat tolerance outcomes. Another key consideration is that the observed improvement in V ̇ O 2 max V_O₂max occurred under thermoneutral conditions (∼16°C), underscoring that these adaptations extend beyond hot environments. This indicates that passive HA can serve as more than a preconditioning measure for upcoming heat exposure, but is also a viable strategy for improving endurance performance across a range of environments. An additional strength of the study was the inclusion of temporal pattern analyses of oxygen transport chain components across the 5 week intervention. Through characterization of which physiological markers first began to change, the researchers provide evidence that meaningful adaptations can emerge in less than 5 weeks. This has important practical implications for athletes and practitioners seeking flexible HA strategies, particularly during periods of heavy training demands or preparation for competition. Jenkins et al. (2025) also identified specific cardiac and haematological adaptations contributing to V ̇ O 2 max V_O₂max improvement, but as endurance performance is multifactorial, additional mechanisms may be involved. For example, HA may promote mitochondrial adaptations (e.g. enhanced bioenergetic function) (Keefe et al., 2025), which could also support observed improvements in performance. Limited research exists regarding the relationship between HA and mitochondrial adaptations; however, enhancements in mitochondrial function have been observed following long-term passive HA in rodents and localized repeated heating to the leg musculature in humans (Keefe et al., 2025). Therefore, future research could investigate potential muscular and metabolic adaptations associated with long-term passive HA that integrate molecular, physiological and performance assessments. The authors also outline several directions for future research. Incorporating erythropoietin measurements would allow clarification on whether downstream responses resemble those seen with altitude training. Additionally, determining the optimal ‘dose’ of heat stress needed to maintain or extend heat adaptations remains another important area of investigation and would support more efficient planning of external heat-stress interventions. Given that 75% of HWI sessions were performed immediately following exercise, where subjects were likely to be in a ‘primed’ state (e.g. elevated core temperature, heart rate), it cannot be certain if the same magnitude of adaptations would be observed if HWI was purposely not performed immediately following training (Steward et al., 2025). Beyond the authors’ recommendations, further considerations arise from the study's limitations. Notably, the absence of a thermoneutral water-immersion control group presents difficulties isolating the effects of heat stress from those of hydrostatic pressure, an influential stimulus in its own right (Weenink & Wingelaar, 2021). As many athletes may choose sauna bathing rather than HWI, assuming heat is the primary driver of adaptation, it is essential to determine whether hydrostatic pressure contributes meaningfully to the observed improvements. Future studies incorporating a thermoneutral immersion control will be key for distinguishing these effects and informing best-practice recommendations. Additionally, although both sauna exposure and HWI, when combined with habitual training, provide a repeated passive heat stimulus capable of inducing classical heat adaptations, sauna exposure may be less likely to induce increases in Hbmass comparable to those observed with HWI. HWI provides a sustained thermal load via conductive heat transfer, producing greater and more consistent elevations in core temperature and cardiovascular strain, probably important for erythropoietic signalling. Therefore, evidence for comparable erythropoietic adaptations following sauna exposure alone remains limited, especially when training load is unchanged. Taken together, the study by Jenkins et al. (2025) demonstrated the effectiveness of a 5 week passive HA protocol in eliciting beneficial haematological (i.e. increased Hbmass, blood volume and LVEDV) and endurance performance (i.e. V ̇ O 2 max V_O₂max and speed at V ̇ O 2 max V_O₂max ) adaptations. Given the advanced training status of participants from the current study, these findings highlight the value of incorporating passive heat exposure into standard training and competition schedules to support physiological adaptations for endurance athletes. Moreover, as meaningful improvements were observed in highly trained individuals, it is reasonable to expect that endurance athletes across a wider range of training statuses may also experience similar benefits from passive HA. Please note: The publisher is not responsible for the content or functionality of any supporting information supplied by the authors. Any queries (other than missing content) should be directed to the corresponding author for the article. No competing interests are declared. All authors contributed to the conception of the work, drafting the work or revising it critically for important intellectual content. All authors approved the final version of the work. None.