Cryptogenic Steatotic Liver Disease: Characterizing a Lean Phenotype Linked to Increased Liver-Related Mortality

Highlights

• Cryptogenic steatotic liver disease (SLD) constitutes a significant subset of lean SLD cases lacking conventional cardiometabolic risk factors (CMRFs).
• This phenotype exhibits distinctive liver injury markers and fibrosis progression, alongside a higher prevalence of genetic risk alleles PNPLA3 and TM6SF2.
• Longitudinal cohorts demonstrate a significant association between cryptogenic SLD and increased liver-related mortality, underscoring its clinical relevance despite absence of overt metabolic dysfunction.
• Findings challenge the common metabolic-centric paradigm of SLD by identifying a vulnerable lean subgroup with unique pathophysiology and prognosis.

Background

Steatotic liver disease (SLD), particularly nonalcoholic fatty liver disease (NAFLD), has been increasingly recognized as a leading cause of chronic liver disease worldwide. Traditionally associated with obesity and metabolic dysfunctions such as diabetes, dyslipidemia, and hypertension — collectively termed cardiometabolic risk factors (CMRFs) — NAFLD manifests as a spectrum ranging from simple steatosis to steatohepatitis and cirrhosis. However, a subset of patients presents with fatty liver changes despite being lean and lacking recorded CMRFs, referred to as “cryptogenic SLD.” This phenotype remains poorly defined and understood, with uncertain pathogenesis and prognostic implications.

Given the global rise in lean fatty liver cases, understanding the clinical characteristics, genetic underpinnings, and outcomes of cryptogenic SLD is critical. This review synthesizes recent evidence, particularly from the study by Yoon EL et al. (2026), which operationalized cryptogenic SLD and systematically assessed its features and outcomes across multiple large cohorts.

Key Content

Defining Cryptogenic Steatotic Liver Disease

The study by Yoon et al. utilized the UK Biobank (UKB) imaging cohort to define SLD as liver fat fraction measured by magnetic resonance proton density fat fraction (MR-PDFF) ≥5%. Within the subset of 30,847 MR-PDFF participants, non-obese individuals (BMI <25 kg/m²) with evidence of hepatic steatosis were classified into two groups: 13.7% cryptogenic SLD (no recorded CMRFs) and 86.3% lean metabolic dysfunction-associated SLD (presence of at least one CMRF). This operational definition provides a rigorous framework to distinguish cryptogenic SLD as lean steatosis absent metabolic syndrome components.

Clinical and Metabolic Profiles Across Cohorts

Comparative analyses across UKB imaging and non-imaging cohorts (including the National Health and Nutrition Examination Survey [NHANES] and Korean National Health Insurance Service [KNHIS]) consistently showed that cryptogenic SLD patients had less favorable liver-related biochemical and imaging parameters compared with non-SLD/no CMRF controls. Despite lacking overt metabolic risk factors, cryptogenic SLD demonstrated markers indicative of hepatic injury and dysfunction, suggesting alternate pathophysiological processes beyond classical metabolic syndrome.

Genetic Predisposition: Risk Variants in PNPLA3 and TM6SF2

A notable finding was the increased prevalence of specific genetic risk alleles within the cryptogenic SLD group. PNPLA3 (patatin-like phospholipase domain-containing protein 3) and TM6SF2 (transmembrane 6 superfamily member 2) variants — both robustly linked to fatty liver disease susceptibility and progression — were enriched in cryptogenic SLD patients in the UKB MR-PDFF cohort. The enhanced expression of these alleles suggests a genetic predisposition that may drive hepatic steatosis and fibrosis independent of metabolic derangements.

Imaging Biomarkers and Fibrosis Assessment

Advanced magnetic resonance imaging modalities revealed higher contrast-enhanced T1-weighted values and increased fibrosis rates by magnetic resonance elastography in cryptogenic SLD compared to lean non-SLD individuals. These findings underscore active liver injury and fibrogenesis despite absence of conventional CMRFs, indicating that cryptogenic SLD is not benign.

Longitudinal Outcomes: Liver-Related Mortality Risk

Importantly, analyses from two independent longitudinal cohorts (UKB hepatic steatosis index-based and KNHIS) demonstrated that cryptogenic SLD was significantly associated with increased risk of liver-related death. In the KNHIS, the hazard ratio (HR) was 2.5 (95% CI 1.4 to 4.3), while the UKB cohort showed a similar, though wider confidence interval estimate (HR 13.2, 95% CI 1.9 to 92.4). These associations persisted after stringent exclusion of alcohol-related liver disease, reinforcing the pathogenic significance of cryptogenic SLD.

Expert Commentary

The identification and characterization of cryptogenic SLD challenge existing conceptual frameworks predominated by metabolic dysfunction as the principal driver of fatty liver disease. Although lean, patients with cryptogenic SLD exhibit significant liver injury and poorer outcomes, implicating genetic and potentially environmental or yet-undiscovered factors contributing to disease pathogenesis.

The enrichment of PNPLA3 and TM6SF2 risk alleles is consistent with prior genetic epidemiology studies pointing to adiponutrin and lipid metabolism-related pathways influencing fat accumulation and fibrogenesis in the liver. These findings highlight a distinct biological substrate that predisposes lean individuals to progressive liver disease, necessitating expanded screening and tailored management strategies beyond traditional risk assessments.

Current clinical guidelines primarily focus on identifying and managing fatty liver in the context of metabolic syndrome and obesity. However, cryptogenic SLD underscores the need for heightened clinical awareness of lean patients with unexplained hepatic steatosis, who may be overlooked due to absence of classic risk profiles but remain at high risk of adverse liver outcomes.

Furthermore, the integration of imaging biomarkers (MR-PDFF, contrast-enhanced T1 mapping, and elastography) proves invaluable for early detection and risk stratification in this population where biopsy is often not feasible. Future studies should elucidate modifiable environmental, dietary, or epigenetic contributors to cryptogenic SLD progression and evaluate the benefit of pharmacologic interventions tailored to the underlying pathophysiology.

Conclusion

Cryptogenic steatotic liver disease, defined as lean hepatic steatosis without recorded cardiometabolic risk factors, accounts for a meaningful proportion of lean SLD cases. This phenotype exhibits distinct clinical, genetic, and imaging features indicative of liver injury and fibrosis progression. Longitudinal evidence implicates cryptogenic SLD as a high-risk subgroup with increased liver-related mortality, emphasizing the imperative for enhanced detection, risk assessment, and research into targeted therapies. These insights broaden the understanding of fatty liver disease beyond metabolic paradigms and call for revised clinical frameworks embracing genetic and phenotypic heterogeneity.

References

• Yoon EL, Lee HY, Lee J, Oh JH, Hwang I, Park CH, Kim S, Park H, Jo AJ, Jun DW. Cryptogenic steatotic liver disease: a lean phenotype associated with increased liver-related mortality. Gut. 2026 Aug 14. PMID: 42463421. https://pubmed.ncbi.nlm.nih.gov/42463421/
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• Sookoian S, Pirola CJ. Meta-analysis of the influence of TM6SF2 E167K variant on nonalcoholic fatty liver disease susceptibility and histological severity. Hepatology. 2015 May;61(5):1113-23. PMID: 25326244.
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