Preoperative Tyrosine Levels as Predictive Biomarkers for Excessive Fat-Free Mass Loss Following Laparoscopic Sleeve Gastrectomy in Patients with Morbid Obesity

<b>Background/Objectives</b>: Fat-free mass (FFM) loss after metabolic and bariatric surgery (MBS) is associated with adverse long-term outcomes, including osteoporosis. Identifying biomarkers that predict excessive FFM loss can improve perioperative patient management and postoperative risk stratification. This study investigated whether preoperative amino acid metabolite (AAM) levels could predict excessive FFM loss after laparoscopic sleeve gastrectomy (LSG). <b>Methods</b>: Forty patients with morbid obesity who underwent LSG between 2019 and 2020 were retrospectively analyzed. Based on the FFM loss to body weight loss ratio (%FFML/BWL) at 3 months postoperatively, patients were categorized into excessive (>25%) and non-excessive (≤25%) FFM loss groups. Anthropometric measurements and serum sampling were performed preoperatively and at 3, 6, and 12 months postoperatively. AAM profiles were collected before surgery. Statistical analyses, including logistic regression and receiver operating characteristic curves, were performed. <b>Results</b>: Twenty-five patients showed excessive FFM loss 3 months after surgery. They had significantly lower preoperative tyrosine (Tyr) levels (<i>p</i> = 0.025). Logistic regression revealed that higher Tyr levels were significantly associated with lower odds of being male, suggesting a potential protective effect (odds ratio (OR) =0.019, <i>p</i> = 0.010). Tyr profiling demonstrated acceptable predictive performance (area under the curve =0.715, <i>p</i> = 0.025). Despite nonsignificant <i>p</i>-values, trends showed lower FFM and muscle mass and higher fat mass in the excessive FFM loss group throughout follow-up. <b>Conclusions</b>: Preoperative Tyr profiling may help identify patients at risk for excessive FFM loss. These findings support prioritizing metabolic health alongside total weight loss in the evaluation of MBS outcomes.