Heat acclimation and sex differences: a crossover study on blood, performance, and health

Abstract

Introduction & Purpose: Heat acclimation (HA) protocols are increasingly used by athletes to optimize performance in both warm and temperate conditions. Benefits may arise from systemic adaptations that enhance thermoregulation and reduce physiological strain at a given workload. These adaptations occur primarily through plasma volume (PV) expansion, which typically develops after 3–4 days of heat exposure. Total hemoglobin mass (Hbmass) was recently shown to increase following PV expansion after 3-5 weeks of HA. However, most research to date has focused on male athletes, particularly regarding circulating blood volume adaptations. Emerging evidence suggests that women may require greater thermal stress or a longer HA duration to obtain similar benefits. Besides, although heat is commonly used to improve exercise performance, its potential additional stimulus for health purposes remains largely unexplored. The present study therefore examined hematological changes, performance outcomes, and body composition in men compared to women following a 10-day indoor training program, with or without additional thermal stress, in a randomized cross-over design.

Methods: Twenty trained amateur cyclists and triathletes (11 men, 9 women) participated in a HA intervention consisting of 10 training sessions over 2 weeks performed either in temperate conditions (CON; 20 °C, 40% relative humidity) or in a heat-controlled environment (HEAT; 35 °C, 60% relative humidity), with at least 3 weeks of wash-out between training blocks. Blood samples were collected at rest before training (PRE), after 3 days (D3), 10 days (D10), and 7 days after the last session (POST) for full blood count analysis (including hemoglobin concentration and reticulocytes percentage) by flow cytometry (Sysmex XN-1000). Circulating blood volumes (i.e., red blood cell volume (RBCV), total blood volume (BV), PV, and Hbmass) were determined using a carbon monoxide rebreathing method (Detalo Performance) at PRE, D10, and POST. Whole-body composition (DEXA), peak oxygen consumption (VO_2peak) during an incremental cycling test (Cortex Metalyzer 3B), and time to complete a 250-kJ time trial were assessed at PRE and POST.

Results: Across all participants, the increase in core temperature was greater in HEAT compared to CON (time × condition interaction, F(9, 323) = 3.23; p < 0.001), reaching peak temperature of 38.8 ± 0.4 °C in HEAT and 38.0 ± 0.5 °C in CON (p < 0.001). Average HR and rate of perceived exertion (RPE) during training progressively declined during the 10 HEAT sessions, reaching significance after the 5th (−9.1 ± 10.2 bpm, p < 0.001) and 3rd sessions (−0.87 ± 1.59 a.u., p = 0.043), respectively. Across all participants, no time × condition interaction was found for blood volumes variables. Nevertheless, the HEAT intervention still led to significant increases at D10 in RBCV (+35 ± 68 mL, p = 0.038), PV (+162 ± 308 mL, p = 0.019), and BV (+197 ± 342 mL, p = 0.005). In parallel, sex-specific analyses revealed time × sex interactions for absolute Hbmass (F(6, 70) = 2.44; p = 0.03) and RBCV (F(6, 70) = 4.43; p < 0.001). In addition, when comparing sexes over time, increases from PRE to D10 were observed in men only for absolute Hbmass (+15.9 ± 21.8 g, p = 0.006), relative Hbmass (+0.25 ± 0.46 g·kg⁻¹, p = 0.022), RBCV (+75 ± 52 mL, p < 0.001), and BV (+259 ± 381 mL, p = 0.009), with RBCV remaining elevated at POST (+45 ± 61 mL, p = 0.017) while the other blood volumes variables return to baseline. Independently of the sex, no concomitant change was reported for hemoglobin concentration (13.8 ± 1.4 vs. 13.6 ± 1.4, p = 0.09) and reticulocytes percentage (1.20 ± 0.25 vs. 1.22 ± 0.26, p = 0.98). The HEAT intervention did not affect VO_2peak (+1.1 ± 2.5 mL·min·kg-1, p = 0.56) or 250-kJ TT time (–82 ± 233 sec, p = 0.081) but resulted in a decrease in visceral adipose tissue (−32 ± 33 g, p < 0.001), with a significant time × condition interaction (F(1,38) = 6.62, p = 0.014). Finally, Hbmass changes were associated with body mass (r = 0.50, p = 0.02), absolute training workload (r = 0.54, p = 0.01), and the ratio of body surface area to body mass (r = −0.56, p = 0.01).

Discussion: The small increase in Hbmass (~16 g) observed in men only is consistent with existing data in elite male cyclists, reporting gains of 30 and 40 g after 3 and 4 weeks of HA, respectively. In line with previous reports of similar Hbmass increases in both sexes after 5 weeks of HA, the present results support the hypothesis that women may need either a longer exposure period and/or a higher absolute thermal load to achieve comparable adaptations. The greater thermal strain typically experienced by men may be a possible explanation for the reported sex-related differences. The significant correlations of Hbmass changes with body mass, absolute training workload, and the ratio of body surface area to body mass, support this interpretation. However, erythropoiesis-related markers, such as reticulocyte percentage, remained unexpectedly stable in both sexes, despite the observed Hbmass increase in men that would typically indicate enhanced erythropoietic activity. Finally, the decrease in visceral adipose tissue following 10 days of HA highlights a potential unexplored clinical application. As visceral fat is a major pathogenic factor underlying metabolic syndrome, incorporating heat exposure alongside exercise may offer a novel therapeutic strategy for overweight individuals.

Conclusion: To the best of our knowledge, this is the first study to show an increase in Hbmass after only 10 days of HA in men. However, this adaptation did not translate into improved endurance performance, and a rapid decay was observed already one week after HA. Our findings also suggest a temporal sex difference in hematological responses to HA, with women potentially requiring a higher or longer thermal stimulus to achieve adaptations similar to men. However, mechanisms underlying Hbmass expansion following HA remain unclear with a limited erythropoietic response observed. Finally, this study highlights potential health implications, as significant changes in body composition were evident after just 10 days of HA.