Beyond body mass index: phenotypic and circulating biomarkers for the identification and characterization of sarcopenic obesity

Sarcopenic obesity (SO), the coexistence of excess adiposity with low skeletal muscle mass and function, is a growing but frequently underdiagnosed condition associated with physical disability, cardiometabolic disease, and increased mortality. Its recognition is hampered by an over-reliance on body mass index, which conceals muscle depletion, and by consensus diagnostic pathways that identify SO only once measurable functional impairment has developed. Advances in body-composition imaging and molecular biology may enable detection of subclinical abnormalities not captured by conventional anthropometric or functional assessment. This narrative review examines the phenotypic and circulating biomarkers proposed for the identification and characterization of SO. It first considers phenotypic markers of muscle quantity and, increasingly, muscle quality including myosteatosis quantified by computed tomography and the bioelectrical phase angle. It then surveys the molecular landscape: myokines and adipokines (myostatin, irisin, the leptin-to-adiponectin ratio), inflammatory mediators (such as tumor necrosis factor-α and interleukin-6) that drive anabolic resistance, and emerging metabolomic, lipidomic, and microRNA signatures. Drawing these together, we propose an integrated phenotype–biomarker framework for precision risk characterization, and consider its relevance in the era of highly effective pharmacological weight loss, where muscle preservation has become an urgent priority. Although most molecular markers remain investigational and lack standardized cut-offs, their integration with phenotypic assessment offers a promising route toward earlier, individualized identification of SO; assay standardization and prospective validation are the principal priorities for translation. The proposed framework is conceptual and has not been prospectively validated.