Association Between Pre-Operative Fat Mass and Appendicular Lean Soft-Tissue Mass Before and One Year After Laparoscopic Sleeve Gastrectomy: A Sex-Stratified Body Composition Study

Background/Objectives: Total weight loss after laparoscopic sleeve gastrectomy (LSG) does not fully capture changes in body composition, particularly the loss of lean mass. We examined sex-specific changes in regional and whole-body composition before and one year after LSG. Methods: This retrospective single-center study included 121 adult patients (84 female, 37 male) who underwent LSG at Istanbul Aydin University Faculty of Medicine between May 2017 and February 2025. Multi-frequency segmental bioelectrical impedance analysis was performed pre-operatively and at one year post-operatively. Three ALM normalization indices (ALM/weight × 100, ALM/BMI, ALM/height2) were compared before and after surgery. Associations between pre-operative fat mass (FM) and ALM were examined using sex-stratified linear regression with and without adjustment for height, age, and sex via ordinary least squares residualization. Results: Pre- and post-operative values were compared. We found that BMI, fat-free mass (FFM), and FM decreased significantly in both sexes (all p < 0.001), with comparable total weight loss between females and males (p > 0.05). Males lost more absolute FFM and showed greater proportional lean mass loss in the limbs, while females showed greater proportional loss in the trunk. After adjusting for height, age, and sex, pre-operative FM was positively associated with ALM both pre-post-operatively (both p < 0.001). Patients with greater pre-operative FM lost more ALM over the surgical year. Conclusions: Body composition changes following laparoscopic sleeve gastrectomy in this cohort of 121 patients were metric-dependent. Patients with lower pre-operative adiposity may present with lower post-operative ALM one year after LSG.