A clinical overview by Dr. Chetan Kalal, DM (Hepatology), Associate Director — Hepatology & Liver Transplant, Gleneagles Hospital, Mumbai
Picture this: you are in your early thirties, your weight is normal, you exercise three times a week, and your last blood test came back unremarkable. Then an abdominal ultrasound — done incidentally during a routine health check — shows something unexpected. Fat in the liver. Fatty liver disease, in a person who does not fit a single conventional risk factor for it.
This scenario is no longer rare. Hepatology clinics across India are seeing a steady increase in exactly this kind of patient. The condition has a name — lean MASLD, part of the broader spectrum now called Metabolic Dysfunction-Associated Steatotic Liver Disease — and understanding why it happens is not an academic question. Without the right explanation, there is no right treatment plan.
The Old Risk Profile No Longer Tells the Whole Story
Fatty liver was historically associated with obesity, type 2 diabetes, and heavy alcohol intake. Those associations are real, but they are not the complete picture. A substantial proportion of patients with confirmed hepatic steatosis carry a normal BMI, have no diabetes, and drink little or no alcohol. In Indian clinical practice, this pattern is common enough that lean MASLD is now considered a distinct and important entity — one that is underdiagnosed precisely because patients and clinicians alike assume that a normal weight rules out the diagnosis.
Why It Happens: The Real Causes
1. Genetics — PNPLA3, TM6SF2, and the South Asian disadvantage
The most significant driver of lean MASLD is genetic. Variants in the PNPLA3 gene impair the liver’s ability to export triglycerides, causing fat to accumulate within liver cells even when dietary intake and body weight are entirely normal. The TM6SF2 variant works through a related mechanism, reducing hepatic lipid secretion. Both are associated with a higher risk of not just fatty liver but also more advanced liver disease — fibrosis and cirrhosis — independent of weight or metabolic status.
These risk variants are substantially more prevalent in South Asian populations than in Europeans. This genetic reality helps explain a pattern that has puzzled many Indian patients: fatty liver diagnosed in a lean, fit individual with no obvious lifestyle risk factor. The liver did not malfunction because of what the patient ate or weighed — the predisposition was written into their genetics from the start.
2. Visceral fat and sarcopenic obesity — the hidden adiposity
BMI measures total body weight relative to height. It does not tell you where the fat is stored. A person with a perfectly normal BMI can carry significant amounts of visceral fat — fat that accumulates around the abdominal organs, rather than under the skin — and visceral fat is metabolically far more harmful than subcutaneous fat. It drives insulin resistance, systemic inflammation, and hepatic fat deposition at levels that body weight alone would not predict.
A related but distinct phenomenon is sarcopenic obesity: low skeletal muscle mass combined with excess body fat, occurring in a person who appears thin or normal. With less muscle available to absorb and metabolise glucose, the liver compensates by converting the excess into fat. The result is fatty liver in someone who looks lean — and who may never be flagged for further investigation because their outward appearance suggests good metabolic health.
3. Insulin resistance before the diabetes label
Many patients with lean MASLD have measurable insulin resistance — meaning their tissues respond poorly to insulin — even when their fasting blood glucose and HbA1c fall within normal limits. This is a pre-diabetic metabolic state that does not yet meet the diagnostic threshold for type 2 diabetes, so it is rarely tested for in the context of a routine health check.
The liver is exquisitely sensitive to insulin signalling. When that signalling is impaired, the liver receives conflicting instructions: it upregulates fat synthesis and simultaneously reduces fat export. The combination produces steatosis in the absence of overt hyperglycaemia — a condition that a standard blood glucose test will entirely miss.
4. Diet quality, not just calorie count
Two people can consume similar total calories and have very different liver outcomes depending on what those calories consist of. Fructose — found in soft drinks, packaged fruit juices, and added sugars in processed food — is metabolised almost exclusively in the liver, and in excess, it is shunted directly into fat synthesis. Refined carbohydrates and ultra-processed foods drive similar hepatic lipogenic pathways.
A person eating what they would describe as a “normal” diet — not overeating, not obviously unhealthy — can still be delivering a daily fructose and refined carbohydrate load that a genetically susceptible liver cannot handle. No visible weight gain is required for this process to cause measurable liver damage over years.
5. Hormonal triggers — hypothyroidism and PCOS
Hypothyroidism reduces the rate at which the liver clears fat; even subclinical hypothyroidism (where thyroid hormone levels are low-normal but TSH is elevated) can be sufficient to cause steatosis. This is reversible with appropriate thyroid treatment, making it one of the most important and under-recognised causes of fatty liver in lean patients — particularly in women.
Polycystic ovarian syndrome (PCOS) is independently associated with insulin resistance and elevated androgens, both of which promote hepatic fat accumulation. Young women with PCOS carry a meaningful risk of developing fatty liver even at normal body weight, and liver assessment is warranted as part of their long-term metabolic monitoring.
6. Medications that cause hepatic steatosis
Several commonly used drugs can cause or worsen fatty liver as a direct effect, regardless of the patient’s baseline metabolic risk. The list includes corticosteroids used for inflammatory conditions, tamoxifen prescribed for breast cancer, valproate for epilepsy, and methotrexate used in rheumatological disease. Patients on any of these agents who develop raised liver enzymes deserve formal hepatological evaluation — not simply a repeat blood test in three months.
7. The gut-liver connection
The gut microbiome communicates directly with the liver via the portal circulation. Dysbiosis — a disturbed balance of gut bacteria driven by low-fibre diet, frequent antibiotic use, chronic psychological stress, and disrupted sleep — increases gut permeability and allows bacterial metabolites to reach the liver. The liver responds with inflammation that compounds steatosis, independent of what else is driving it. This mechanism can contribute to fatty liver in patients whose diet and lifestyle otherwise appear unremarkable.
Why It Gets Missed
Lean MASLD is invisible in its early stages. There is no pain, no jaundice, no swelling, no clinical sign that prompts investigation. Liver enzyme tests — ALT and AST — are frequently normal or only mildly elevated in the early years of the disease; a normal result is still routinely taken as reassurance that the liver is fine, which it is not. The disease can progress through inflammation to fibrosis without producing a single abnormal blood test value that would trigger further investigation under standard clinical practice.
The combination of a normal BMI, normal blood sugar, and normal liver enzymes creates a false sense of security. By the time symptoms appear — fatigue, discomfort, or the incidental finding of a firm liver edge — the disease has often been silent for years.
Who Should Be Screened
Anyone with the following should consider a liver assessment, irrespective of body weight or blood sugar:
- A close family member with fatty liver disease, cirrhosis, or liver cancer
- Hypothyroidism or PCOS, particularly if suboptimally treated
- Long-term use of corticosteroids, methotrexate, tamoxifen, or antiepileptic drugs
- Persistently elevated ALT or AST without an identified cause
- Metabolic syndrome features — central adiposity, elevated triglycerides, low HDL — even with normal BMI
- A dietary pattern consistently high in fructose, refined carbohydrate, or ultra-processed food
Screening begins with liver enzyme tests and an abdominal ultrasound. When steatosis is confirmed, the next step is fibrosis assessment — most practically done with a FibroScan (transient elastography), which measures liver stiffness non-invasively. Current APASL guidance on MASLD recommends staged fibrosis assessment in all patients with confirmed steatosis, not only those in the traditionally defined high-risk categories. The reason is straightforward: even lean patients without overt metabolic disease can harbour advanced fibrosis, and identifying it early changes the clinical management fundamentally.
What To Do If Your Scan Shows Fatty Liver
Finding steatosis on imaging is not a reason to panic. Caught early, fatty liver is reversible with the right lifestyle changes — reducing refined sugar and processed carbohydrate intake, increasing dietary fibre and protein, and building consistent physical activity. When a specific cause is identified — hypothyroidism, a culprit medication, early insulin resistance — treating that cause often resolves the liver finding without requiring anything more complex.
What you should not do is dismiss it. If a scan has flagged fatty liver in someone who is lean, non-diabetic, and otherwise “healthy,” the appropriate response is a proper hepatological evaluation: fibrosis staging, a metabolic screen including fasting insulin and lipid profile, thyroid function, and a structured dietary review. The cause matters for the treatment, and lean MASLD — precisely because it is unexpected — is the form most likely to be undertreated.
This article is written for general health education. It is not a substitute for individual medical assessment. If you have concerns about your liver health, please consult a qualified hepatologist.