**Background:** Bariatric surgery is effective for sustained weight loss in severe obesity, but the long-term time course of body composition changes—particularly lean tissue mass (LTM) versus fat mass (FM) loss—remains unclear. Most studies have limited follow-up to 12 months and focus on Roux-en-Y gastric bypass rather than sleeve gastrectomy (SG), which has become the most common bariatric procedure. Visceral adipose tissue (VAT) is particularly relevant due to its role in lipotoxicity, insulin resistance, and inflammation, yet few studies have evaluated VAT changes after SG. This study aimed to characterize FM, LTM, and VAT changes from the acute phase (1 month) through weight stabilization (12 and 24 months) following SG, along with biological parameters and resting energy expenditure (REE).
**Methods:** From November 2016 to July 2020, 83 Caucasian patients (63 women, 75.9%; mean age 40.9 ± 12.3 years; mean BMI 40.7 ± 4.2 kg/m²) were recruited from the obesity management centre at University Hospital of Montpellier. Inclusion criteria were BMI > 40 or BMI ≥ 35 kg/m² with comorbidities, and failure of other weight loss treatments. All patients underwent SG. Body composition was measured by dual-energy X-ray absorptiometry (DXA) at baseline and 1, 12, and 24 months post-surgery, including whole-body and regional FM and LTM, as well as abdominal total adipose tissue (TAT), VAT, and subcutaneous adipose tissue (SAT). Blood samples were collected for glucose homeostasis (glucose, insulin, HbA1c, HOMA-IR), lipid profile, C-reactive protein (CRP), IGF-1, IGFBP-3, and albumin. REE was measured by indirect calorimetry. Sarcopenia was assessed using EWGSOP2, FNIH, and IWGS criteria. Statistical analyses used paired Wilcoxon or Student's tests, with p < 0.05 considered significant.
**Key Results:** At 1 month, relative weight loss was −9.1 ± 2.1%, with LTM loss of −9.6 ± 4.0% and FM loss of −7.9 ± 3.5%. At 12 months, weight loss reached −29.3 ± 8.4%, with LTM loss of −19.4 ± 6.5% and FM loss of −42.7 ± 12.5%. At 24 months, weight loss was −27.5 ± 9.6%, with LTM loss of −19.5 ± 7.1% and FM loss of −38.2 ± 14.6%. No significant changes occurred between 12 and 24 months. VAT decreased by −7.16 ± 18.45% at 1 month, −34.80 ± 35.12% at 12 months, and −38.40 ± 26.08% at 24 months. SAT decreased by −7.98 ± 8.36%, −46.89 ± 18.23%, and −39.40 ± 19.93% at the same time points. HOMA-IR decreased from 8.1 ± 7.1 at baseline to 2.3 ± 2.3 at 12 months (−66.2 ± 23.8%) and 2.5 ± 1.9 at 24 months (−65.5 ± 21.6%). HbA1c decreased from 5.9 ± 1.0% to 5.4 ± 0.5% at 12 months (−6.4 ± 8.4%) and 5.6 ± 0.6% at 24 months (−5.9 ± 9.3%). CRP decreased from 7.3 ± 5.7 mg/mL to 2.1 ± 2.2 mg/mL at 12 months (−66.4 ± 32.4%) and 1.9 ± 2.4 mg/mL at 24 months (−64.7 ± 55.6%). IGF-1 initially decreased at 1 month (−10.8 ± 24.1%) but increased above baseline at 12 months (+12.5 ± 23.3%) and 24 months (+16.0 ± 29.7%), while IGFBP-3 decreased throughout (−12.3 ± 14.4% at 1 month, −15.3 ± 11.8% at 12 months, −12.9 ± 16.7% at 24 months). REE decreased by −15.7 ± 10.0% at 1 month, −23.2 ± 8.3% at 12 months, and −24.0 ± 9.3% at 24 months. Sarcopenia prevalence did not increase post-surgery; at 24 months, only 2 patients met sarcopenia criteria by FNIH/EWGSOP definitions. Age was positively but weakly correlated with LTM and FM loss at 12 and 24 months.
**Clinical Implications:** This study demonstrates that after SG, LTM loss occurs predominantly in the first month, while FM loss is more progressive over 12 months, with stabilization thereafter. The first postoperative month is a critical window for LTM preservation, suggesting that early interventions—such as protein supplementation and resistance training—may help maintain LTM and REE, potentially reducing the risk of weight regain. The improvement in metabolic parameters (HbA1c, HOMA-IR, lipids, CRP) and the increase in IGF-1 above baseline at 12 and 24 months, alongside reduced IGFBP-3, suggest a recovery of the somatotropic axis that may support LTM preservation. VAT reduction (−38.4% at 24 months) is clinically significant given its association with cardiovascular risk and insulin resistance. The absence of increased sarcopenia prevalence post-surgery is reassuring, though the maintenance of REE remains a concern for long-term weight management. Routine body composition assessment, including VAT measurement, should be considered in post-bariatric care.