Weight-Loss Magnitude, Fat Mass, and Device-Estimated Muscle Mass Loss During Obesity Pharmacotherapy

Background: Body-composition changes during routine obesity pharmacotherapy require cautious interpretation. Methods: This retrospective observational cohort included 59 adults with paired InBody 770 assessments: 53 receiving tirzepatide and 6 receiving semaglutide. Paired changes and relative device-estimated muscle mass loss were analyzed in all 59; component-share analyses were restricted to the 51 achieving ≥5% weight loss. Analyses used rank-based methods and exploratory robust regression. Results: Recorded treatment duration was a median of 7.0 months, and median observed weight loss was 12.11%. Median fat mass and device-estimated muscle mass losses were 9.9 and 2.3 kg, respectively. Among the 51 participants achieving ≥5% weight loss, median fat, device-estimated muscle, and residual shares were 73.95%, 14.91%, and 10.85%. Each additional five percentage points of weight loss was associated with 2.47 percentage points greater relative device-estimated muscle mass loss (95% CI, 1.89 to 3.05). The corresponding fully adjusted association with muscle share was −0.52 percentage points (95% CI, −1.66 to 0.62); this does not establish stability or equivalence. Final drug-specific dose intensity was not statistically associated with either outcome in the adjusted models. Conclusions: Weight loss observed in this selected, predominantly tirzepatide-treated cohort was mainly accounted for by estimated fat mass reduction. Larger observed weight-loss responses accompanied greater device-estimated muscle mass reductions. Muscle-share estimates were uncertain and model-dependent. The study cannot establish causal medication effects or functional consequences.