Oxygen as a Metabolic Modulator: Divergent Physiological Adaptations to Hypoxic and Hyperoxic Tabata Training

Background: Whether inspired oxygen fraction (FiO₂) can modulate the internal meta-bolic cost of supramaximal high-intensity interval training (HIIT) and thereby direct training adaptation remains unclear. We tested whether hyperoxic versus hypoxic ex-posure during Tabata-format HIIT induces distinct adaptive phenotypes. Methods: Twenty-three physically active men completed 3 weeks of supramaximal Tabata HIIT (3 sessions·week⁻¹; 8 × 20 s with 10 s recovery) under hyperoxia (FiO₂ = 0.60, n = 13) or hypoxia (FiO₂ = 0.16, n = 10). Training intensity was regulated to main-tain a comparable internal physiological stimulus rather than an identical external workload. Pre- and post-intervention assessments included maximal oxygen uptake (VO₂max), first and second ventilatory thresholds (VT1, VT2), peak blood lactate, and session rating of perceived exertion (RPE). Post-intervention between-group differences were analysed using ANCOVA adjusted for baseline values; RPE was analysed using a linear mixed-effects model. Results: VO₂max improved in both groups but increased more after hyperoxic training than after hypoxic training (+3.69 vs. +1.50 mL·kg⁻¹·min⁻¹; β = 2.18 mL·kg⁻¹·min⁻¹, 95% CI [1.77–2.59], p < 0.001). Hyperoxia also produced larger gains in VT1 (β = 29.99 W, 95% CI [17.09–42.89], p < 0.001) and VT2 (β = 20.74 W, 95% CI [9.43–32.05], p = 0.001). Peak lactate responses diverged bidirectionally, decreasing in hyperoxia (−0.77 mmol·L⁻¹) and increasing slightly in hypoxia (+0.27 mmol·L⁻¹), with a significant ad-justed between-group effect (β = −1.02 mmol·L⁻¹, 95% CI [−1.47 to −0.57], p < 0.001). RPE declined across sessions in both groups, with a steeper decrease under hyperoxia (Con-dition × Session: β = −0.36, 95% CI [−0.44 to −0.28], p < 0.001). Conclusions: Hyperoxic and hypoxic supramaximal HIIT elicited distinct functional adaptive profiles. Hyperoxia induced greater improvements in aerobic capacity and ventilatory thresholds, reduced peak lactate accumulation, and accelerated the decline in perceived exertion, whereas hypoxia was associated with a more glycolytic response pattern. These findings support the interpretation that FiO₂ acts as a modulator of in-ternal physiological load and shapes the metabolic phenotype of adaptation during su-pramaximal interval training.