Daily hot-water immersion preserves altitude-induced haemoglobin mass expansion following descent independent of erythropoietin

High-altitude exposure increases haemoglobin mass (Hb_mass), a key determinant of arterial oxygen-carrying capacity, but following descent this adaptation can regress toward baseline within 7 days. Long-term heat acclimation has emerged as an alternative stimulus for Hb_mass expansion; however, whether post-altitude passive-heat exposure can preserve altitude-induced increases in Hb_mass remains unclear. Furthermore, the extent to which any preservation of Hb_mass is supported by sustained erythropoietin (EPO) production, and whether this support is mediated by plasma volume expansion or acute alterations in renal haemodynamics, has yet to be explored. Twenty-one healthy adults (8F) sojourned at 3800 m for 14 days and, following descent to 1250 m, were allocated to either hot-water immersion (HWI; 45 min at 40°C for 4 days; n = 11) or control (CON; n = 10). Hb_mass, intravascular volumes (carbon monoxide rebreathing) and circulating EPO were measured on high-altitude day 1 (HA1), day 14 (HA14), and post-descent day 5 (P5). Renal-artery blood velocity (Doppler-ultrasound) was measured before and after HWI on the day of descent. Hb_mass increased by 24 g [95% CI: 8, 40] across all participants during the altitude sojourn (P = 0.005). Following descent, Hb_mass decreased in CON (∆ = −18 g [95% CI: −36, 0], P = 0.045) but was maintained in HWI (∆ = +9 g [95% CI: −8, 26], P = 0.285). Circulating EPO declined after descent (P < 0.001) with no between-condition difference at P5 (P = 0.239), despite a transient reduction in renal-artery blood velocity following HWI (P = 0.025) and similar plasma volume expansion across groups (time: P < 0.001). Hot-water immersion offers a practical, lower-impact alternative to exercise-heat methods for preserving altitude-derived Hb_mass expansion, although the mechanisms underlying this response remain elusive.