Background: Body mass index (BMI) is the most widely used proxy of adiposity in clinical and epidemiological practice, yet its relationship with cardiovascular efficiency—the capacity of the heart and circulation to deliver and utilise oxygen per unit of physiological cost—remains incompletely characterised. 
Objective: This narrative review synthesises evidence on how BMI relates to cardiorespiratory fitness (VO₂max), cardiac structure and function, and cardiovascular outcomes. 
Methods: Peer-reviewed cohort studies, cross-sectional analyses, meta-analyses and reviews published between 1989 and 2025 were retrieved from PubMed/MEDLINE, Scopus and Web of Science and synthesised narratively. 
Findings: Higher BMI is consistently associated with lower VO₂max, the association being stronger in women and for central-adiposity measures. Elevated BMI drives volume overload, left ventricular hypertrophy and diastolic dysfunction, while higher fitness substantially attenuates mortality risk and frequently outperforms BMI as a prognostic marker. After established cardiovascular disease, an “obesity paradox” of improved survival is observed. 
Conclusion: BMI is a useful but imperfect index of cardiovascular efficiency; integrating cardiorespiratory fitness and body-composition measures refines risk stratification.