Effects of High-Intensity Interval Training on Cardiorespiratory Fitness and Cardiometabolic Health in Real-World...

High-intensity interval training (HIIT) is an effective exercise strategy for improving cardiorespiratory fitness (CRF) and cardiometabolic health in laboratory settings, yet its effectiveness in real-world conditions remains less clear. This systematic review and meta-analysis examined the effects of real-world HIIT on CRF and cardiometabolic outcomes in adults. We searched five major databases (inception to December 2025) for controlled and non-controlled reports of ≥1 cardiorespiratory or cardiometabolic outcome. Random-effects models were used to calculate weighted mean differences (WMD), with subgroup and meta-regression analyses to explore moderators. Thirty-one studies (n = 1119) were included. In the pooled pre-post analyses, HIIT was associated with significant within-group improvements in VO_2max/peak (3.9 mL/kg/min), systolic (-4 mmHg) and diastolic blood pressure (-3 mmHg), triglycerides (-8 mg/dL), total cholesterol (-9 mg/dL), LDL (-10 mg/dL), and waist circumference (-1.7 cm) across interventions. In controlled comparisons, HIIT was superior to passive controls for VO_2max/peak (6.9 mL/kg/min) and waist circumference (-2.9 cm), while superiority over active controls was limited to VO_2max/peak (1.1 mL/kg/min). Moderator analyses revealed larger cardiometabolic benefits in overweight and clinical populations, and greater improvements in fasting glucose and blood pressure with higher exercise volume and longer intervention duration. In conclusion, real-world HIIT can effectively improve CRF and selected cardiometabolic outcomes across diverse adult populations. Specific cardiometabolic benefits appear to be pronounced in overweight, clinical, and previously untrained individuals. Notably, substantial heterogeneity and moderate methodological quality warrant cautious interpretation.