Short-Term Associations Between Fat-Free Mass Preservation and Glycaemic Markers During Tirzepatide Therapy: A Secondary Exploratory Analysis

Background/Objectives: Tirzepatide (TZP), a dual glucose-dependent insulinotropic polypeptide and glucagon-like peptide-1 receptor agonist, induces substantial weight loss in patients with obesity; however, pharmacologically induced weight reduction may be accompanied by losses in fat-free mass (FFM), muscle strength (MS), and resting metabolic rate (RMR), potentially influencing metabolic health. The metabolic implications of short-term preservation of metabolically active tissue during TZP therapy remain incompletely characterized. Methods: We performed a secondary, exploratory analysis of a previously published 12-week prospective, non-randomized comparative study including 60 patients with obesity treated with TZP (n = 30 TZP+Low Energy Ketogenic Therapy [LEKT]; n = 30 TZP+Low Calorie Diet [LCD]). Body weight (BW), fat mass (FM), FFM, MS, and RMR were assessed at baseline and week 12. Glycaemic parameters included fasting glucose, insulin, hemoglobin A1c (HbA1c), and HOMA-IR. All analyses were exploratory and hypothesis-generating. Results: Both groups achieved comparable reductions in BW after 12 weeks. FM decreased in both groups, while relative preservation of FFM, MS, and RMR was observed in one dietary context. Short-term changes in HbA1c, insulin, and HOMA-IR were statistically associated with concurrent changes in FFM, MS, and RMR, whereas no consistent associations were observed with changes in total BW or FM. Baseline glycaemic values were largely within the normoglycemic range. Conclusions: In this short-term secondary exploratory analysis, preservation of metabolically active tissue during TZP therapy was associated with concurrent glycaemic profiles, whereas no consistent associations were observed with total weight loss magnitude. These findings do not imply causality and should be interpreted as hypothesis-generating, warranting confirmation in larger, randomized, long-term studies.