Mechanisms of haemoglobin mass expansion following heat stress

Haemoglobin mass is a critical determinant of oxygen delivery to working muscle, with even modest increases enhancing maximal oxygen uptake ( ${\dot{V}}_{O_2\max }$ ) and endurance performance. While altitude training has long been utilised to increase haemoglobin mass, its efficacy is increasingly debated, due to concerns over maladaptation, inter-individual variability, cost and environmental impact. More recently, heat training - traditionally employed for acclimatisation ahead of competition in warm climates - has emerged as a feasible long-term stimulus capable of eliciting comparable haemoglobin mass expansion. Long-term heat acclimation (≥5 weeks) shows promise as an environmental intervention to improve oxygen-carrying capacity via haematological adaptation, with each study published to date reporting meaningful (∼2-4%) haemoglobin mass expansion and concurrent improvements in ${\dot{V}}_{O_2\max }$ . Erythropoiesis underpins the haematological adaptations to long-term heat acclimation, though it appears to diverge from established hypoxia-driven mechanisms.  This review will discuss how heat exposure may stimulate erythropoietin via distinct cellular stress signalling, altered renal oxygen tension or plasma volume perturbations. These responses may act through distinct pathways, complementing or deviating from those traditionally associated with hypoxia. While published long-term heat acclimation studies have primarily utilised exercise under heat stress conditions, we discuss the potential for passive heating methods to yield similar adaptations. Based on our review of the literature, we highlight the need for research that elucidates precise mechanisms, compares differing modes of heat stress, and explores broader applications of long-term heat acclimation. However, current evidence supports long-term heat acclimation as an effective alternative or adjunct to altitude training for enhancing oxygen-carrying capacity.