Carbon monoxide supplementation: evaluating its potential to enhance altitude training effects and cycling performance in elite athletes.

Altitude training is a cornerstone for endurance athletes for improving blood variables and performance, with optimal effects observed at ∼2,300-2,500 meters above sea level (m.a.s.l.). However, elite cyclists face challenges such as limited access to such altitudes, inadequate training facilities, and high expenses. To address these issues, a novel method involving daily exposure to carbon monoxide (CO) has been proposed to amplify altitude training adaptations at suboptimal altitudes. Thirty-one male cyclists were assigned to three groups: Live-High Train-High with CO inhalation (LHTH_CO), Live-High Train-High (LHTH), and Live-Low Train-Low (LLTL). The LHTH_CO group underwent CO inhalation twice daily in the afternoon/evening to elevate carboxyhemoglobin concentration to ∼10%. Hematological variables, in vivo muscle oxidative capacity, and physiological indicators of cycling performance were assessed before and after a 3-week altitude training camp at 2,100 m.a.s.l. LHTH_CO demonstrated a larger increase in hemoglobin mass (Hb_mass) compared to both LHTH and LLTL. Although there were no statistical differences between LHTH_CO and LHTH in submaximal and maximal performance measures, LHTH_CO displayed greater improvements in 1-min maximal power output during incremental testing (W_max), power output at lactate threshold, and maximal oxygen consumption (V̇o_2max) compared to LLTL. LHTH demonstrated a larger improvement than LLTL in W_max and V̇o_2max, with no group differences in Hb_mass or submaximal measures. Muscle oxidative capacity did not differ between groups. These findings suggest that combining moderate-altitude training with daily CO inhalation promotes hematological adaptations more effectively than moderate altitude alone and enhances cycling performance metrics in cyclists more than sea-level training.NEW & NOTEWORTHY Three weeks of training at moderate altitude with exposure to low doses of CO can significantly enhance hematological adaptations in elite cyclists compared to moderate-altitude training alone. Cycling performance determinants improved more with CO inhalation at moderate altitude compared to sea-level training, whereas there were no differences in submaximal and maximal performance measures compared to moderate-altitude training alone. This study highlights the potential of CO supplementation as an effective adjunct to altitude training regimens.