Effects of High-Intensity Interval Training on Body Composition and Cardiometabolic Health in Physically Inactive Individuals: A Systematic Review and Meta-Analysis of Randomized Controlled Trials

Background/Objectives: Increasing physical activity is important for reducing cardiometabolic risk. However, evidence on the effects of high-intensity interval training (HIIT) in physically inactive individuals, including those with extremely sedentary behavior, remains inconsistent. We evaluated the effects of HIIT on body composition and cardiometabolic outcomes compared with a non-exercise control (CON) and moderate-intensity continuous training (MICT). Methods: PubMed, Web of Science, Embase, the Cochrane Library, Ovid-hosted resources, and ClinicalTrials.gov were searched through 8 July 2026. The protocol was registered (PROSPERO CRD420251230848). Random-effects models were used. Risk of bias, evidence certainty, and robustness were assessed using the Cochrane Risk of Bias 2 (RoB 2) tool, Grading of Recommendations Assessment, Development and Evaluation (GRADE), and leave-one-out analyses, respectively. Results: Twenty-four randomized trials involving 877 participants were analyzed. A conservative study-level summary classified all studies as having at least some concerns, including five at high risk of bias. Compared with CON, HIIT reduced body weight (weighted mean difference WMD = −1.60 kg), systolic blood pressure (WMD = −2.85 mmHg), diastolic blood pressure (WMD = −6.57 mmHg), low-density lipoprotein cholesterol (WMD = −0.29 mmol/L), and total cholesterol (WMD = −0.42 mmol/L), and increased maximal oxygen uptake (VO2max; WMD = 7.72 mL/kg/min). Compared with MICT, most outcomes showed no statistically significant differences, but HIIT produced a greater VO2max increase (WMD = 3.72 mL/kg/min). Several findings were sensitive to individual-study removal, and certainty of evidence was low or very low. No serious adverse events were reported in 11 studies; attendance or adherence was generally high in 15 studies, despite incomplete reporting and supervised interventions. Conclusions: HIIT may improve selected body composition and cardiometabolic outcomes, with VO2max showing a relatively consistent direction of effect. However, the certainty of evidence for most outcomes was low or very low; therefore, these findings should be interpreted with caution. Current evidence is insufficient to establish equivalence to MICT or identify an optimal HIIT protocol. Larger, longer-term, high-quality trials using objective measures of sedentary behavior are needed, particularly in extremely sedentary populations.