BACKGROUND AND OBJECTIVE: Weight regain following metabolic and bariatric surgery (MBS) remains a major clinical challenge, particularly among individuals with metabolic abnormalities. This study aimed to evaluate the effect of preoperative very low-energy diet (VLED) on 1-year postoperative weight loss success and to explore its potential metabolic mediators. METHODS: In this prospective observational cohort study, 248 patients undergoing laparoscopic sleeve gastrectomy were allocated to either a VLED group (n = 124) or a non-VLED group (n = 124) with matched baseline characteristics. The primary outcome was defined as a ≥25% reduction in body mass index (BMI) at 12 months postoperatively. Subgroup, Cox regression, and mediation analyses were conducted to assess the impact of VLED and identify potential mediators including HbA1c, triglycerides (TG), and high-density lipoprotein cholesterol (HDL-C). RESULTS: Patients in the VLED group demonstrated significantly higher rates of weight loss success (58.9% vs. 37.1%, P = 0.014) and greater improvements in metabolic parameters (HbA1c, HOMA-IR, TG, HDL-C). Cox regression revealed that VLED was associated with a lower risk of weight loss success failure (HR=0.62, P = 0.004), particularly among individuals with HOMA-IR >2.5. Mediation analysis showed that HbA1c reduction accounted for 31.1% of VLED's effect on weight loss success, followed by TG and HDL-C. CONCLUSION: Preoperative VLED significantly enhances weight loss success after bariatric surgery, especially in patients with insulin resistance, with metabolic improvements in HbA1c, TG, and HDL-C partially mediating the beneficial effects. These findings support incorporating VLED into preoperative care for high-risk patients.