Association of low body mass index with the risk of major adverse cardiovascular events among patients with acute myocardial infarction: a single-center cohort study from Iran

The association between body mass index (BMI) and outcomes after acute myocardial infarction (AMI) remains controversial. While obesity is a known cardiovascular risk factor, low BMI may also have prognostic implications. This study aimed to investigate the association between BMI categories and major adverse cardiovascular events (MACE) in AMI patients. This retrospective cohort study included 1,018 AMI patients admitted to Afshar Hospital, Yazd, Iran. Patients were categorized into four BMI groups: underweight (< 18.5 kg/m²), normal weight, overweight, and obese. Demographic information along with risk factors, clinical history, and clinical presentation, were extracted. MACE was defined as all-cause mortality, AMI, ischemic stroke, cardiovascular hospitalizations, and revascularization procedures. Univariate and multivariate logistic regression analyses and Restricted cubic spline (RCS) regression method with four knots were used to assess the relationships between BMI and outcomes. Underweight patients (11.3%) had significantly higher mortality (24.3%) compared to other BMI groups (P < 0.001). After adjustment, underweight remained an independent risk factor for mortality (OR 3.12, 95% CI 1.42–6.89, p = 0.005), but not for MACE. No significant associations were found between overweight or obesity and increased risk of MACE or mortality. The dose-response analysis revealed a U-shaped curve for mortality (P for nonlinearity < 0.001), but not for MACE (P = 0.12). While obesity is a well-established cardiovascular risk factor, our findings suggest that low BMI is associated with increased one-year mortality in patients with AMI, but not with MACE after adjustment. These results should be interpreted with caution, as low BMI may reflect underlying health status rather than a direct causal factor. Further studies are needed to clarify the mechanisms underlying this association.