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

Altitude training is a well-established method for increasing hemoglobin mass (Hb_mass) in endurance athletes, with altitudes >2,300-2,500 m 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 (1,865 m.a.s.l.) could enhance hematological adaptations. Forty-six well-trained cross-country skiers and biathletes [F, 20; M, 26; maximal oxygen uptake, 69.1 (8.2) mL·min^-1·kg^-1] were assigned to one of three groups for a 3 wk 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). Hematological 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 with 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 hematological 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.NEW & NOTEWORTHY Adding three weekly heat training sessions to a training camp at moderate natural altitude (<2,000 m.a.s.l.) resulted in greater increase in hemoglobin mass compared with traditional training at moderate altitude or sea level in well-trained endurance athletes. However, these hematological improvements were not accompanied by significant between-group differences in submaximal or maximal roller-ski performance indicators. Heat training may represent a practical strategy for coaches and athletes to enhance erythropoietic adaptations when higher-altitude environments are unavailable.