Effects of hypoxic high-intensity interval training on cardiorespiratory fitness, body composition, and cardiometabolic outcomes in non-clinical populations: a systematic review and meta-analysis with an age-related evidence distribution assessment

Hypoxic high-intensity interval training (HHIIT) has been proposed as a time-efficient strategy for enhancing physiological adaptations. However, its efficacy in non-clinical populations remains inconsistent. This systematic review and three-level meta-analysis followed the PRISMA 2020 guidelines. PubMed, Web of Science, Scopus, and Embase were searched from database inception to 20 December 2025. Fourteen randomized controlled trials, including 534 participants, met the eligibility criteria. HHIIT produced a significant, small positive effect on overall cardiorespiratory fitness outcomes (Hedges' g = 0.369, 95% CI [0.186, 0.552], P < 0.001), and the results remained robust following CR2 small-sample correction. For relative oxygen uptake, HHIIT showed a moderate beneficial effect (g = 0.553, 95% CI [0.308, 0.797], P < 0.001). Subgroup analyses indicated that the effects of HHIIT were larger when compared with non-exercise controls than with exercise controls. n contrast, HHIIT had no significant overall effect on body composition or cardiometabolic outcomes (g = - 0.018, 95% CI [- 0.133, 0.097]) or on hematological outcomes (g = 0.122, 95% CI [- 0.292, 0.535]). The current evidence base is concentrated mainly in young adults, with limited representation of females, middle-aged adults, and older adults. In conclusion, low-to-moderate certainty evidence suggests that HHIIT may provide limited, modality-specific improvements in cardiopulmonary adaptations in non-clinical populations, particularly in relative oxygen uptake. However, substantial intervention heterogeneity warrants cautious interpretation of these findings. Its effects on body composition, cardiometabolic health, and hematological parameters remain unclear. Future trials should standardize hypoxic dose and training prescriptions, while addressing existing age- and sex-related evidence gaps. These efforts are needed to clarify the generalizability and practical value of HHIIT across broader populations.