Type 1 Diabetes Mellitus as a Model of Body Composition Transformation: Pathogenetic Unity of Combined Phenotypes

The study of body composition (BC) is of increasing clinical and research relevance, as it provides a detailed characterization of metabolic status beyond traditional anthropometric indices such as body mass index (BMI). BC describes the quantitative ratio of fat mass (FM), lean mass (LM)—including skeletal muscle and internal organs—and bone mineral density (BMD). The pathogenetic mechanisms underlying specific BC phenotypes arise from a complex interplay of endocrine, metabolic, inflammatory, and genetic factors. Given the shared pathways involved in phenotype formation, the study of BC alterations across different diseases holds promise for predicting complications and identifying common therapeutic targets. This review aims to systematize current knowledge on combined BC phenotypes—sarcopenic obesity, osteosarcopenia, osteopenic obesity, and osteosarcopenic obesity—and to explore their pathogenetic unity, with particular emphasis on type 1 diabetes mellitus as a dynamic model of sequential phenotypic shifts.