Purpose: To investigate the association between serum amino acid changes and alopecia in patients with obesity before and after bariatric surgery, and identify risk factors for postoperative alopecia.Methods: This prospective cohort study (ChiCTR2300074104) included patients with obesity undergoing laparoscopic sleeve gastrectomy (LSG), categorized into non-mild alopecia (NM group, n = 24) and moderate–severe alopecia (MS group, n = 43) groups. Clinical data and serum amino acid concentrations were analyzed to: (1) compare preoperative and postoperative amino acid profiles; (2) evaluate amino acid differences between groups at various timepoints and correlate with alopecia severity; (3) perform logistic regression to identify alopecia risk factors.Results: Among 67 patients analyzed, LSG significantly decreased serum concentrations of arginine, alanine, threonine, glutamic acid, branched-chain amino acids (valine, isoleucine, leucine), and aromatic amino acids (tyrosine, phenylalanine, tryptophan) (p < 0.05), while serine and glycine increased (p < 0.05). At 3 months postoperatively, leucine levels were higher in the MS group compared to the NM group. Spearman’s correlation analysis revealed threonine, γ-aminobutyric acid, and leucine levels were associated with alopecia severity (p < 0.05). Logistic regression identified serum leucine as an independent risk factor for postoperative alopecia (OR = 1.119, 95% CI 1.006-1.245, p = 0.038).Conclusion: LSG surgery alters the serum amino acid profile in patients with obesity, and serum leucine concentration at 3 months postoperatively is an influential factor in postoperative alopecia.