Type 5 diabetes mellitus: nutritional-imprinted β-cell insufficiency, diagnostic gaps, and emerging therapeutic strategies

Historical descriptions of malnutrition-related diabetes mellitus (MRDM) have regained attention in contemporary discourse, with the term type 5 diabetes mellitus (T5DM) increasingly used as a harmonized research construct to describe insulin-deficient diabetes associated with early-life undernutrition. Although the <i>International Classification of Diseases, 11th Revision</i> (ICD-11) includes categories related to malnutrition-associated diabetes, the pathophysiological interpretation, diagnostic boundaries, and therapeutic implications of T5DM remain incompletely defined, resulting in variable clinical adoption. Consequently, many affected individuals continue to be classified as lean type 2 diabetes and managed within obesity-centric care paradigms. This narrative review synthesizes existing evidence to advance a conceptual framework that distinguishes T5DM from other lean diabetes phenotypes by integrating developmental nutritional reserve, β-cell functional capacity, and autoimmune status. Clinical observations are highlighted to indicate that body size-based descriptors alone may not adequately capture clinically relevant nutritional heterogeneity. Emerging pharmacological evidence is also reviewed, suggesting that responses to glucagon-like peptide-1 receptor agonists, sodium-glucose cotransporter-2 inhibitors, and insulin secretagogues may not be determined solely by residual β-cell functional reserve, but may also be modulated by underlying nutritional status. Building on this premise, a nutrition-integrated pharmacometabolic model is proposed as a hypothesis-generating framework, in which standardized nutritional assessment is considered alongside β-cell evaluation to inform exploratory treatment stratification rather than prescriptive clinical guidance. Finally, priorities for translational research are outlined, including the development of consensus-based indicators of nutritional reserve, improved phenotypic classification frameworks, and nutritional reserve-stratified randomized trials. Conceptualizing T5DM as a nutritionally conditioned metabolic phenotype may facilitate more targeted research agendas and support context-sensitive, individualized interventions in populations where malnutrition and diabetes intersect.