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In patients with major depressive disorder (MDD), the relationship between body mass index (BMI) and risk of suicide attempts (SA) remains unclear, and findings are controversial. The aim of this study was to investigate the relationship between BMI and SA in a Chinese population with first-episode drug-naïve (FEDN) MDD. A total of 1718 patients with FEDN MDD were enrolled in this cross-sectional study. Their socio-demographic characteristics as well as anthropometric data were collected. The 17-item Hamilton Rating Scale (HAMD-17) and Hamilton Anxiety Scale (HAMA) were used to assess the severity of depression and anxiety symptoms in all participants. Thyroid hormones, lipid profile, and fasting blood glucose (FBG) were measured. A history of suicide attempts was verified based on medical records and interviews with patients and their family members. Multiple logistic regression analysis was used to estimate the association between BMI and the risk of SA. Threshold effects were examined by a two-piecewise logistic regression model. Multiple logistic regression analysis showed that BMI was independently and negatively correlated with SA (OR = 0.91, 95%CI: 0.85 to 0.98, P = 0.01) in patients with FEDN MDD after adjusting for covariates. Smoothing plots also revealed a non-linear (L-shaped) relationship between BMI and SA, and a two-piecewise logistic regression model was used to calculate the inflection point of BMI as 22.1 kg/m2. On the left of the inflection point, a negative association between BMI and SA was detected (OR = 0.54, 95%CI: 0.42 to 0.70, P < 0.001), while no significant association was observed on the right side of the inflection point (OR = 1.01, 95%CI: 0.93 to 1.10, P = 0.75). Our results suggest that lower BMI is associated with a higher risk of recent SA in Chinese patients with FEDN MDD, especially in those with BMI below 22.1 kg/m2. • BMI inversely correlated with suicide attempts in drug-naïve MDD patients (adjusted for covariates). • Non-linear BMI-SA relationship: negative association on left (BMI < 22.1 kg/m2), no association with BMI >= 22.1 kg/m2.2. • Underweight positively linked to SA, overweight negatively linked. No evidence of obesity-SA association. [ABSTRACT FROM AUTHOR]