MASLD versus MAFLD in lean steatotic liver disease: diagnostic overlap, inclusivity, and the metabolically healthy lean phenotype.

Lean non-alcoholic fatty liver disease (NAFLD) presents a unique clinical phenotype increasingly recognized within the evolving nomenclature of metabolic dysfunction–associated fatty liver disease (MAFLD) and metabolic dysfunction–associated steatotic liver disease (MASLD). This study evaluated the diagnostic applicability of both frameworks in lean Egyptian patients and explored their capacity to encompass metabolically healthy lean phenotypes. Ninety ultrasonography confirmed lean NAFLD patients (BMI < 25 kg/m²) were reassessed using MAFLD and MASLD criteria. Clinical, metabolic, and biochemical parameters, alongside non-invasive fibrosis indices and transient elastography, were compared across classifications. Among all patients, 87.8% met MASLD and 77.8% fulfilled MAFLD criteria, showing substantial overlap yet minor classification differences. MASLD identified a slightly higher proportion of lean cases, whereas MAFLD more stringently captured those with overt metabolic dysfunction. Clinical, biochemical, and fibrosis parameters were comparable among groups (p > 0.05). However, 12.2% of patients remained unclassified under either definition, representing a metabolically healthy lean with steatotic liver disease (MHL-SLD) subgroup. In lean steatotic liver disease, MASLD and MAFLD show substantial diagnostic overlaps and identify similar patient profiles with no significant metabolic or fibrosis differences. MASLD includes slightly more patients, yet without clear clinical distinction. Refinement of criteria may be needed to better capture metabolically healthy lean individuals with liver steatosis (MHL-SLD).