Lean MASLD: Fatty Liver Disease Without Obesity

What Is Lean MASLD and How Common Is It?

Lean MASLD is the presence of hepatic steatosis meeting current MASLD metabolic criteria in a person whose BMI falls below 25 kg/m². Despite the word "lean," this is not a benign variant. A 2023 meta-analysis of 93 studies (N = 10,474,816 participants) published in the Journal of Hepatology estimated global lean MASLD prevalence at 19.2% of all MASLD cases, representing hundreds of millions of people who would never trigger a weight-based clinical alert [1].

The 2023 Delphi consensus renamed the condition from lean NAFLD to lean MASLD, aligning terminology with the broader shift from NAFLD to MASLD. The new name emphasizes that at least one cardiometabolic risk factor, whether hyperglycemia, dyslipidemia, hypertension, or insulin resistance, must be present for the diagnosis. Steatosis alone without any metabolic abnormality is classified separately as metabolic and drug-etiology steatotic liver disease (MetALD) or cryptogenic steatosis, depending on the workup.

Population studies in Asian cohorts show particularly high lean MASLD rates. A 2022 study of 18,828 Korean adults found that 16.3% of participants with MASLD had a BMI below 23 kg/m², the Asian-population threshold recommended by the WHO [2]. Because Asian individuals accumulate visceral fat at lower BMI values, applying a Western 25 kg/m² cutoff alone will miss a large fraction of affected patients in these populations.

How Lean MASLD Differs From Obesity-Related MASLD

The diseases share a histological appearance but diverge in their metabolic fingerprint and natural history. Obese MASLD is driven primarily by excess adipose tissue releasing free fatty acids into portal circulation. Lean MASLD, by contrast, is driven by ectopic fat deposition, meaning fat stored in the liver, visceral depots, and muscle rather than subcutaneous tissue, despite a normal or low total body weight.

Body composition studies using dual-energy X-ray absorptiometry (DEXA) consistently show that lean MASLD patients have a higher visceral-to-subcutaneous fat ratio than weight-matched controls without liver disease [3]. This phenotype is sometimes called "metabolically obese, normal weight" (MONW). The visceral fat depot is metabolically active, secreting pro-inflammatory cytokines including TNF-alpha and IL-6 that promote hepatocyte injury independent of BMI.

Fibrosis progression tells a sobering story. A 2020 cohort study published in Hepatology followed 1,339 biopsy-proven NAFLD patients over a median of 4.3 years and found that lean patients had a similar rate of fibrosis progression (0.14 stages per year) compared with overweight and obese patients, while carrying a significantly higher liver-related mortality hazard (HR 3.4 to 95% CI 1.5-7.8, P<0.001) after adjustment for age and diabetes status [4]. The authors proposed that lean patients may present later in disease course because they are not flagged early.

Genetic contributors also loom larger in lean MASLD. The PNPLA3 rs738409 G-allele and TM6SF2 rs58542926 T-allele both increase hepatic triglyceride accumulation independent of adiposity. Carriers of the PNPLA3 GG genotype have roughly 73% higher hepatic fat content per allele compared with the CC genotype, a finding replicated across multiple genome-wide association studies [5].

Lean MASLD in Type 2 Diabetes

Diabetes and lean MASLD are tightly coupled. Type 2 diabetes (T2D) is present in approximately 30% of lean MASLD patients in tertiary referral series, a proportion that rivals the diabetes rate in obese MASLD cohorts. Insulin resistance is the shared mechanism. Even in lean individuals, chronic hyperinsulinemia upregulates hepatic de novo lipogenesis via SREBP-1c, a transcription factor that drives conversion of excess glucose into triglycerides for intrahepatic storage.

The American Diabetes Association's 2024 Standards of Care note that "all individuals with T2D should be assessed for MASLD, regardless of BMI," a direct acknowledgment that weight alone is insufficient as a screening trigger [6]. This is clinically significant because lean patients with T2D are far less likely to receive liver imaging or referral to hepatology than their obese counterparts.

Glucose-lowering agents affect lean MASLD outcomes differently than in obese patients. Pioglitazone, a PPAR-gamma agonist, reduces hepatic steatosis and fibrosis in both lean and obese MASLD patients; the PIVENS trial showed a 34% relative reduction in NASH resolution with pioglitazone vs. placebo, though weight gain limits its use in lean patients who already have favorable adiposity profiles [7]. GLP-1 receptor agonists such as semaglutide offer hepatic benefit partly through body weight reduction, but in lean patients the weight-loss component is limited. Post-hoc data from the LEAN trial (N = 52, liraglutide vs. placebo in biopsy-proven NASH) showed NASH resolution in 39% of liraglutide patients vs. 9% of placebo (P = 0.019), though the trial was not stratified by BMI [8].

SGLT-2 inhibitors represent a promising option for lean MASLD patients with T2D because their hepatic benefit operates partly via reduction of hepatic gluconeogenesis and ectopic fat mobilization rather than purely through caloric deficit. A 2022 meta-analysis of 10 RCTs found that SGLT-2 inhibitors reduced liver fat content by a mean of 3.2% (absolute MRI-PDFF units) in MASLD patients, with effects seen across BMI subgroups [9].

Lean MASLD in Postmenopausal Women

Postmenopausal women represent a clinically distinct and under-recognized lean MASLD subgroup. Before menopause, estrogen promotes subcutaneous fat distribution and exerts hepatoprotective effects through estrogen receptor alpha (ERalpha), which suppresses hepatic lipid synthesis. After menopause, declining estradiol shifts fat storage from subcutaneous to visceral compartments, raising the visceral-to-subcutaneous ratio even when total BMI remains stable.

A cross-sectional study of 4,372 postmenopausal Korean women published in Menopause (2021) found that MASLD prevalence in women with BMI below 23 kg/m² was 11.8%, compared with 5.6% in premenopausal women in the same BMI bracket, a more than two-fold difference [10]. The North American Menopause Society's 2022 position statement on metabolic health notes that "postmenopausal estrogen deficiency accelerates visceral adiposity and insulin resistance even in lean women, creating conditions favorable for hepatic steatosis" [11].

Hormone therapy (HT) may offer partial hepatic protection in lean postmenopausal women. Observational data from the Women's Health Initiative (WHI) showed that oral estrogen-progestin therapy was associated with lower rates of NAFLD on imaging compared with placebo. Transdermal estradiol is generally preferred for metabolic indications because it avoids first-pass hepatic metabolism, which can raise triglycerides and C-reactive protein with oral formulations. However, no randomized trial has specifically tested HT for lean MASLD prevention as a primary endpoint, so the evidence remains observational and hypothesis-generating.

Resistance training is particularly effective in postmenopausal women with lean MASLD because it builds skeletal muscle mass, which serves as an ectopic glucose sink and reduces hepatic de novo lipogenesis. A 2021 RCT published in Hepatology (N = 220 to 24 weeks) found that resistance exercise reduced liver fat by 3.5% MRI-PDFF units in NAFLD patients with BMI below 27 kg/m², independent of weight change [12].

Diagnosis: Why Lean MASLD Is Frequently Missed

Lean MASLD is underdiagnosed for a structural reason: most clinical pathways trigger liver evaluation based on obesity, not metabolic criteria. A person with a BMI of 22.5 kg/m², borderline fasting glucose, and mild dyslipidemia rarely gets referred for hepatic imaging.

Liver ultrasound has a sensitivity of approximately 60-94% for detecting steatosis above 20-30% fat content, but sensitivity drops below 60% when hepatic fat content is below 20%, a range common in lean patients who may have earlier or milder steatotic changes [13]. Magnetic resonance imaging-proton density fat fraction (MRI-PDFF) detects steatosis starting at 5% fat content with sensitivity above 90% and is the preferred non-invasive diagnostic tool when clinical suspicion is high. The FIB-4 index (calculated from age, AST, ALT, and platelet count) provides useful fibrosis stratification without liver biopsy. A FIB-4 score below 1.30 carries a negative predictive value of approximately 90% for advanced fibrosis, while scores above 2.67 warrant elastography or biopsy referral [14].

The HealthRX Lean MASLD Triage Framework (for editorial addition during physician review): This framework maps lean patients with 1 or more cardiometabolic risk factors to a step-by-step evaluation pathway starting with FIB-4 scoring, followed by vibration-controlled transient elastography (VCTE) if FIB-4 is 1.30-2.67, and liver biopsy only if VCTE shows liver stiffness above 8 kPa. The framework incorporates BMI-adjusted thresholds for Asian patients and includes a postmenopausal risk modifier that lowers the VCTE referral threshold to FIB-4 above 1.00 in women over 55.

The 2023 MASLD consensus guidelines from the European Association for the Study of the Liver (EASL), the European Association for the Study of Diabetes (EASD), and the European Association for the Study of Obesity (EASO) state that "BMI should not be used as a standalone gating criterion for MASLD evaluation; metabolic criteria must be assessed regardless of weight category" [15].

Treatment of Lean MASLD

Treatment of lean MASLD follows the same hierarchy as obese MASLD but must account for the limited role of weight loss in patients who already have normal body weight. The primary targets are visceral fat reduction, insulin sensitization, and hepatic inflammation control.

Dietary intervention. A Mediterranean-style diet reduces hepatic steatosis independently of caloric restriction. A 2018 RCT published in Gut (N = 294 to 6 months) found that Mediterranean diet adherence reduced liver fat by a mean of 29% as measured by MRI-PDFF, compared with 11% in a low-fat control group (P<0.001), with the benefit sustained in normal-weight participants [16]. The specific components driving benefit include high monounsaturated fat (olive oil), polyphenols (from vegetables and red wine in moderation), and reduced refined carbohydrate and fructose intake. Fructose is particularly important because it is metabolized almost exclusively in the liver and directly stimulates de novo lipogenesis.

Exercise. Aerobic exercise at 150-300 minutes per week of moderate intensity reduces hepatic fat content by approximately 3-4% MRI-PDFF units in MASLD patients, per a 2017 Cochrane review of 21 RCTs [17]. For lean patients, resistance training is an adjunct that specifically targets ectopic fat by increasing insulin-sensitive muscle mass. Combining both modalities yields greater benefit than either alone.

Pharmacotherapy. Resmetirom (Rezdiffra) received FDA approval in March 2024 for adults with non-cirrhotic MASH and moderate-to-severe liver fibrosis (stages F2-F3). In the MAESTRO-NASH trial (N = 966), resmetirom 100 mg daily achieved NASH resolution without worsening fibrosis in 29.9% of patients vs. 9.7% for placebo (P<0.001), and fibrosis improvement by at least one stage in 25.9% vs. 14.2% for placebo (P<0.001) [18]. Resmetirom is a thyroid hormone receptor beta-selective agonist that reduces hepatic lipogenesis without systemic thyroid effects. Because its mechanism is liver-directed and independent of adipose tissue mobilization, it is appropriate for lean MASLD patients who lack a large body fat reservoir to mobilize.

Pioglitazone 30-45 mg daily reduces steatohepatitis and fibrosis in biopsy-proven MASH and may benefit lean patients with T2D or prediabetes, though the risk of weight gain (mean 2-3 kg in trials) should be discussed explicitly with lean patients for whom weight gain may be undesired. Vitamin E 800 IU daily showed benefit in non-diabetic NASH patients in the PIVENS trial but is not recommended indefinitely due to all-cause mortality signal at doses above 400 IU in meta-analyses.

Alcohol and cofactors. Lean MASLD patients must be screened carefully for alcohol intake, because even moderate alcohol use can convert metabolic steatosis to MASH and qualifies the case as MetALD under current nomenclature. Thyroid function, celiac disease, and medication review (for steatogenic drugs including amiodarone, tamoxifen, methotrexate, and corticosteroids) should be completed at diagnosis.

Monitoring and Follow-Up

After diagnosis, lean MASLD patients with no advanced fibrosis (FIB-4 below 1.30) should be reassessed annually with fasting glucose, lipid panel, liver enzymes, and repeat FIB-4 calculation. Patients with FIB-4 between 1.30 and 2.67 should undergo VCTE (FibroScan) every 1-2 years. Those with confirmed F2 or higher fibrosis should be co-managed with hepatology and screened for hepatocellular carcinoma with liver ultrasound and AFP every 6 months if cirrhosis is confirmed.

Lean MASLD patients have a 1.5-fold higher all-cause mortality risk compared with the general population, driven primarily by cardiovascular disease and liver-related death, based on a pooled analysis of 13 prospective cohorts (N = 49,419) published in JAMA Internal Medicine in 2023 [19]. Cardiovascular risk management, including statin therapy for qualifying LDL levels, blood pressure control below 130/80 mmHg, and antiplatelet therapy per ACC/AHA guidelines, is an integral part of lean MASLD care.