A 45 min bout of hot‐water immersion induces transient plasma volume contraction without a detectable increase in circulating erythropoietin

Heat acclimation is increasingly used to induce haematological adaptations and enhance endurance performance. Although repeated hot-water immersion (HWI) has been shown to increase haemoglobin mass, the acute physiological responses that might underpin these longer-term adaptations remain poorly understood. This study investigated the 24 h time course of plasma volume (PV) and erythropoietin (EPO) responses following a single bout of HWI. Sixteen healthy adults (four females) completed a randomised crossover study comprising 45 min of HWI (40°C) and a time-matched control condition. Changes in PV were estimated from haemoglobin concentration and haematocrit, while serum EPO concentrations were determined using an ELISA. HWI induced a substantial contraction in PV immediately after immersion (-9.5%, 95% CI [-12.5, -6.5]; P < 0.001), accompanied by marked increases in tympanic temperature and thermal strain. However, PV returned to baseline within 12 h, and no subsequent PV expansion was observed. Serum EPO concentration was unaffected by time (P = 0.183), condition (P = 0.125) or the condition × time interaction (P = 0.309). Furthermore, changes in PV were unrelated to changes in circulating EPO in both the HWI (β = 0.07; [-1.3, 1.4]; P = 0.915) and control (β = 1.10; [-1.1, 3.3]; P = 0.326) conditions. These findings demonstrate that a 45 min bout of HWI induces a substantial but transient contraction in PV that is not accompanied by a measurable increase in circulating EPO within 24 h. Acute PV contractions following HWI therefore appear insufficient to stimulate a measurable erythropoietic response, suggesting that the haematological adaptations observed following chronic heat acclimation may instead depend upon sustained alterations in PV arising from repeated heat exposures.