Influence of Body Mass Index on Perioperative Outcomes of Oncologic Head and Neck Free Flaps: Comprehensive Analysis Using a Prospective Institutional Database

Background: The impact of body mass index (BMI) extremes on the outcomes after head and neck cancer (HNC) reconstruction remains uncertain. Herein, we investigate the influence of low and high BMI on the intraoperative and postoperative outcomes of oncologic head and neck microvascular reconstruction. Methods: We analyzed a prospective institutional database (09/2019–12/2024). Patients were stratified into underweight, normal weight, overweight, and obesity I, II, and III. Flaps were categorized by donor site (thigh, back, fibula, or forearm). Demographics, intraoperative events, and donor, recipient, and systemic postoperative complications were assessed. Results: Among 542 patients, BMI distribution was: 9.6% underweight (<18.5 kg/m<sup>2</sup>), 34.7% normal weight (≥18.5 kg/m<sup>2</sup>, <25 kg/m<sup>2</sup>), 29.5% overweight (≥25 kg/m<sup>2</sup>, <30kg/m<sup>2</sup>), and 15.5%, 7.0%, and 3.7% within obesity I (≥30 kg/m<sup>2</sup>, <35 kg/m<sup>2</sup>), II (≥35 kg/m<sup>2</sup>, <40 kg/m<sup>2</sup>), and III (≥40 kg/m<sup>2</sup>), respectively. Flaps employed significantly differed between low and high BMI groups (<i>P</i><0.001). Underweight patients demonstrated increased odds of donor-site (OR 4.60; <i>P</i>=0.011) and recipient-site (OR 2.49; <i>P</i>=0.007) complications, noteworthily salivary leaks (24.0% vs ≤11.2%; <i>P</i>=0.0322). In subgroup analyses, obesity II patients undergoing thigh-based reconstructions saw increased flap loss (16.7% vs ≤2.4%, <i>P</i>=0.0002); obesity II and III patients trended towards increased recipient-site hematoma (16.7% vs ≤3.6%, <i>P</i>=0.1376) and dehiscence (20.0% vs ≤7.1%, <i>P</i>=0.0824), respectively. <b>Conclusion:</b> BMI extremes relate to perioperative risk in HNC microvascular reconstruction. Underweight status is associated with higher perioperative complications across flaps, while obesity-related risk concentrates in class II/III patients undergoing thigh-based flaps. Findings support BMI-informed preoperative optimization and flap selection strategies.