Body Composition Changes After Bariatric Surgery: Magnitude, Timing, and Determinants of Excessive Fat-Free Mass Loss

<b>Background:</b> Bariatric surgery effectively treats severe obesity, but postoperative weight loss includes reductions in both fat mass (FM) and fat-free mass (FFM). Excessive FFM loss may increase the risk of sarcopenia, frailty, and long-term weight regain, yet its magnitude and determinants are not fully established. <b>Methods:</b> We conducted a retrospective analysis of a prospectively collected cohort of 179 patients who underwent laparoscopic or robotic Roux-en-Y gastric bypass between January 2020 and December 2022. Anthropometric parameters and body composition (bioelectrical impedance analysis) were measured preoperatively and at 6 and 12 months. The proportion of FFM loss relative to total weight loss (%FFML/WL) was calculated, and excessive FFM loss was defined using published cut-offs (≥25%, ≥30%, and ≥35%). Predictors of FFM preservation were assessed through stepwise regression. <b>Results:</b> Baseline BMI was 44.1 ± 4.6 kg/m², FM 54.6 ± 10.7 kg, and FFM 61.1 ± 11.9 kg. At 6 and 12 months, BMI decreased to 31.0 ± 4.2 and 28.8 ± 4.4 kg/m², respectively; FM decreased to 35.6 ± 11.0 and 22.0 ± 10.0 kg; and FFM to 54.7 ± 9.5 and 50.1 ± 7.0 kg (all <i>p</i> < 0.001). Most FFM loss occurred within the first 6 months (mean - 6.4 kg). Median %FFML/WL was 26.4% at 6 months and 28.7% at 12 months. Excessive FFM loss affected 41-46% of patients (≥25%), 27-31% (≥30%), and 14% (≥35%). In multivariable analysis, FFM at 6 months was the only independent predictor of FFM at 12 months (<i>p</i> < 0.001). <b>Conclusions:</b> Bariatric surgery leads to substantial FM and FFM reductions, with nearly half of patients exceeding established %FFML/WL alert thresholds. Early postoperative body composition monitoring may help identify individuals at higher risk of FFM depletion and guide preventive strategies such as adequate protein intake and resistance training.