Lean type 2 diabetes: the overlooked epidemic reshaping global health.

Lean type 2 diabetes mellitus an overlooked epidemic emerges as a distinct clinical entity affecting individuals who maintain normal body mass index according to population-adjusted criteria, yet develop diabetes through pathways independent of obesity. This phenotype accounts for 3-8% of global diabetes cases but demonstrates striking regional clustering, with substantially elevated rates across South Asian, Sub-Saharan African, and Caribbean populations. The epidemic's defining characteristics centre on predominant pancreatic β-cell failure rather than classical insulin resistance mechanisms. Affected individuals exhibit compromised insulin production capacity and altered adipose tissue distribution while maintaining seemingly healthy body weights. Paradoxically, clinical outcomes often prove more severe than obesity-associated diabetes, with earlier disease onset and accelerated progression to major complications including cardiovascular disease, nephropathy, and retinopathy. Current healthcare frameworks systematically fail this population through obesity-focused screening algorithms that overlook lean individuals at risk. Diagnostic delays compound the problem, while conventional treatment protocols emphasizing weight reduction prove not only ineffective but potentially harmful. The epidemic remains largely invisible within existing clinical guidelines and public health strategies. This review synthesizes evidence on lean diabetes and T2DM heterogeneity. Observational studies support population-specific risk assessment and β-cell preservation strategies, yet proposed mechanistic distinctions rely on BMI categories rather than standardized measurements of β-cell function or insulin sensitivity. These expert consensus recommendations require randomized trial validation before guideline incorporation. Treatment algorithms should be guided by mechanistic characterization, not BMI alone. Implementation requires consensus measurement thresholds and validation that treatment responses differ by mechanistic phenotype. Systematic investigation may reduce diagnostic delays and advance health equity in vulnerable populations, though clinical implementation depends on generating rigorous controlled trial evidence.