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.