Tirzepatide, a dual glucose-dependent insulinotropic polypeptide and glucagon-like peptide-1 receptor agonist, has shown significant efficacy for weight loss in individuals with overweight and obesity. However, the implications of this weight reduction on lean mass remain unclear. This narrative review aims to evaluate the current evidence regarding tirzepatide’s impact on body composition, particularly focusing on changes in lean mass and its proxy measures.
A structured literature search identified ten studies, including randomized controlled trials, observational studies, and modeling analyses, which encompassed diverse populations with and without type 2 diabetes mellitus. Across these studies, tirzepatide consistently produced large, dose-dependent reductions in body weight. Notably, these reductions were predominantly attributable to fat mass, although decreases in lean mass, fat-free mass, or skeletal muscle mass were observed in all studies.
The proportion of total weight loss attributable to lean mass generally ranged from 10% to 30%, which aligns with expected physiological responses to weight loss. However, the substantial heterogeneity in study design, population characteristics, and body composition assessment methods limits the ability to draw definitive conclusions. Most studies relied on indirect measures such as bioelectrical impedance analysis, with limited use of dual-energy X-ray absorptiometry.
Importantly, functional outcomes were not assessed, and the clinical significance of the observed reductions in lean mass remains uncertain. Overall, while tirzepatide appears to promote preferential fat loss, some loss of lean mass occurs. Strategies such as resistance training and adequate protein intake may help mitigate potential adverse effects on muscle mass during treatment.
What this study does not show
This review does not establish a direct causal relationship between tirzepatide use and specific outcomes related to lean mass, nor does it assess functional outcomes related to muscle health.
Evidence scorecard
- Analytic validity: partial
- Clinical validity: partial
- Clinical utility: not established