Long-term passive heat acclimation enhances maximal oxygen consumption via haematological and cardiac adaptation in...

Long-term heat acclimation has been shown to elevate haemoglobin mass (Hb_mass) and enhance maximal oxygen consumption ( ${\dot{V}}_{O_2\max }$ ). To date, however, all evidence derives from exercise-induced heat stress, making it unclear whether passive strategies, which allow athletes to maintain training intensities, can produce similar adaptations. Moreover, while heat-induced increases in ${\dot{V}}_{O_2\max }$ are often attributed to haematological changes, adaptation of other convective components of oxygen transport may underpin these improvements. To investigate whether 5 weeks of hot-water immersion (HWI) would enhance ${\dot{V}}_{O_2\max }$ , 10 well-trained runners (nine male; ${\dot{V}}_{O_2\max }$  = 64.5 ± 8.1 mL min^-1 kg^-1) completed a within-subject, counterbalanced cross-over intervention involving HWI (5 × 45 min week^-1; ≥40°C) and a time-matched control, alongside habitual training. Haematological and cardiac adaptations were assessed using carbon-monoxide rebreathing (in duplicate) and four-dimensional (4D) speckle-tracking echocardiography, respectively. HWI increased Hb_mass (+33 g, P < 0.001; [95% confidence interval (CI) 18 to 49]), blood volume (+284 mL, P < 0.001; [95% CI 113 to 455]) and left ventricular end-diastolic volume (LVEDV; +10 mL, P < 0.001; [95% CI 6 to 13]), without altering 4D global peak longitudinal strain, diastolic-strain rate or diastolic-filling rate. These changes facilitated a 2.7 mL kg^-1 min^-1 improvement in ${\dot{V}}_{O_2\max }$ (P < 0.001; [95% CI 1.4 to 4.1]) and a 0.8 km h^-1 increase in treadmill speed at ${\dot{V}}_{O_2\max }$ (P < 0.001; [95% CI 0.5 to 1.1]). Best subset regression analysis identified Hb_mass as the strongest independent predictor of ${\dot{V}}_{O_2\max }$ (β = 3.8, P < 0.001), with cardiac adaptations contributing additional explanatory value (R² = 0.825 fixed effects; 0.961 full model). Our findings demonstrate that passive heat exposure elicits coordinated adaptations to multiple convective components of the oxygen transport chain, and represents a physiologically effective and sustainable strategy for enhancing ${\dot{V}}_{O_2\max }$ in well-trained populations. KEY POINTS: Long-term heat acclimation can increase haemoglobin mass and maximal oxygen uptake; whether passive heat can produce similar effects was previously unknown. Whether cardiac adaptation contributes to the heat-induced improvements in maximal oxygen consumption remains unclear. In a within-subject, counterbalanced cross-over design, 10 well-trained runners completed 5 weeks of hot-water immersion alongside their normal training and a time-matched control period. Passive heating increased haemoglobin mass, total blood volume, and left-ventricular end-diastolic volume. These haematological and cardiac adaptations explained the observed improvement in maximal oxygen uptake, indicating that passive heat can enhance aerobic performance via coordinated effects across multiple components of the oxygen transport chain.