Heat training augments haemoglobin mass during a moderate-altitude training camp in well-trained endurance athletes

Altitude training is a well-established method for increasing haemoglobin mass (Hb mass ) in endurance athletes, with altitudes >2300-2500 meters above sea level (m.a.s.l.) commonly used to promote such adaptations. However, logistical constraints, including limited access to sport-specific facilities (e.g., shooting ranges for biathletes) may limit its use. Therefore, this study investigated whether adding heat training to a camp conducted at moderate natural altitude (1865 m.a.s.l.) could enhance haematological adaptations. Forty-six well-trained cross-country skiers and biathletes (♀=20; ♂=26, maximal oxygen uptake=69.1 (8.2) mL∙min -1 ∙kg -1 ) were assigned to one of three groups for a three-week intervention: 1) Residing and training at moderate altitude with three weekly heat-suit cycling sessions (MOD-ALT heat ), 2) Residing and training at moderate altitude (MOD-ALT), or 3) Residing and training at sea level (CON). Haematological variables and roller-ski performance were assessed before and after the intervention. MOD-ALT heat demonstrated a greater increase in Hb mass (2.9%) compared to both MOD-ALT (1.2%, p=0.017) and CON (-0.2%, p<0.001). The greater Hb mass increase was accompanied by a larger increase in red blood cell volume. No significant difference in Hb mass change was observed between MOD-ALT and CON, and no between-group differences were found in submaximal or maximal performance measures. These findings suggest that including three weekly heat-suit cycling sessions to a moderate-altitude training camp enhances haematological adaptations compared with moderate altitude or sea-level training alone, although no significant between-group differences were observed in roller-skiing performance indicators. This strategy may offer a practical and effective alternative for athletes and coaches when access to higher-altitude training environments is limited.