Fatty liver disease: a third of adults have it, and the stage is what matters

Start here: what MASLD is

The liver stores and processes fat as part of its normal job. When too much accumulates inside liver cells — steatosis — and that accumulation is tied to metabolic dysfunction rather than to alcohol, viral hepatitis or a drug, the result is MASLD: metabolic dysfunction-associated steatotic liver disease.

Most of it does nothing for a long time. A minority of people go further: the fat is accompanied by inflammation and liver-cell injury, which is MASH — metabolic dysfunction-associated steatohepatitis — and it is MASH that lays down scar tissue. Scarring, or fibrosis, is the liver's generic wound-healing response to chronic injury ^[1], and is graded from F0 (none) through F1, F2, F3 to F4, cirrhosis ^[2]. Cirrhosis brings liver failure, portal hypertension and a raised risk of liver cancer.

The names changed recently, and for a reason. Until 2023 these conditions were called NAFLD and NASH — non-alcoholic fatty liver disease and non-alcoholic steatohepatitis. A modified Delphi process run by three pan-national liver associations, with 236 panellists from 56 countries, found that 74% thought the existing nomenclature was flawed enough to justify a change; the word "non-alcoholic" was felt to be stigmatising by 61% of respondents and "fatty" by 66%. The problem was also logical: naming a disease by what it is not forces every diagnosis through an exclusion. Steatotic liver disease was adopted as the umbrella term, "steatohepatitis" was deliberately retained as a pathophysiological concept, and the replacement for NAFLD became metabolic dysfunction-associated steatotic liver disease, redefined positively as steatosis plus at least one of five cardiometabolic risk factors. A new category, MetALD, covers people who have both metabolic dysfunction and higher alcohol intake ^{[3] [4] [5] [6]}. Most of the literature cited below predates the rename and says NAFLD/NASH; the conditions are the same.

The scale is the part people find hard to believe. A meta-analysis of 86 studies covering 8,515,431 people in 22 countries put global NAFLD prevalence at 25.24% (95% CI 22.10–28.65) — roughly a quarter of the world's adults, and higher in the Middle East and South America. Among those with the condition, 51.3% had obesity, 22.5% type 2 diabetes, 69.2% hyperlipidaemia, 39.3% hypertension and 42.5% metabolic syndrome ^[7]. Later estimates put the figure closer to a third of adults, and the economic and clinical burden has been quantified for the United States and Europe ^[8].

Why it matters, and why it is under-recognised. MASLD is usually silent. It produces no symptoms until fibrosis is advanced, it is often found incidentally on an ultrasound done for another reason, and liver enzymes can be entirely normal. Meanwhile it is becoming a leading indication for liver transplantation, and it carries a raised risk of hepatocellular carcinoma ^[7] — including, unusually, in people who have not reached cirrhosis, which is what makes deciding whom to screen so difficult ^[9].

Why it is in the news. For decades there was no approved drug. In 2024 that changed: resmetirom, an oral liver-directed thyroid hormone receptor beta agonist, met both primary endpoints of a phase 3 trial in MASH with fibrosis ^[2] and became the first therapy approved for the condition.

Its neighbours in this series. MASLD sits inside the cardiometabolic cluster already covered here: obesity and diabetes are its principal drivers ^[7], and — as Pillar 2 explains — the commonest cause of death in MASLD is not liver disease at all but cardiovascular disease, which is the subject of the heart failure review.

Three pillars follow — measurements, medicines, and progress — with a fibrosis-progression model between the first two that explains why one number matters more than all the others.

Pillar 1: measurements and diagnosis

The spectrum, and the one variable that predicts outcome

MASLD is best understood as a ladder rather than a diagnosis: steatosis → steatohepatitis (MASH) → fibrosis F1–F3 → cirrhosis (F4). People move up it slowly, some not at all, and a few move back down.

The clinically decisive fact is that fibrosis stage — not the amount of fat, not the inflammation score — is what predicts death. A meta-analysis of five cohort studies covering 1,495 patients and 17,452 patient-years quantified it. Using fibrosis stage 0 as the reference, all-cause mortality rate ratios rose with stage: F1 1.58 (95% CI 1.19–2.11), F2 2.52 (1.85–3.42), F3 3.48 (2.51–4.83), F4 6.40 (4.11–9.95). Liver-related mortality rose far more steeply — F1 1.41, F2 9.57, F3 16.69, F4 42.30 — leading the authors to conclude that "the risk of liver-related mortality increases exponentially with increase in the stage of fibrosis" ^[10]. Those confidence intervals are very wide at the upper stages, and the review's figure draws them rather than hiding them.

This is why the whole diagnostic apparatus below exists to answer one question: how much scarring is there?

Liver biopsy: the reference standard, and its limits

Biopsy remains the reference for staging fibrosis and for grading steatohepatitis, using the NAFLD activity score — a 0–8 composite of steatosis, lobular inflammation and hepatocellular ballooning, which is what trials use as an endpoint ^[2]. It is also invasive, carries a small risk, samples perhaps one fifty-thousandth of the organ, and suffers meaningful sampling and observer variability. Those limitations have been assessed explicitly ^[11]. With a quarter of the adult population potentially affected, biopsying everyone was never an option.

Non-invasive scores: triage, not diagnosis

The practical solution is a two-step approach in which cheap tests rule people out.

FIB-4 is calculated from age, the two common liver enzymes and platelet count — no extra test required. Simple non-invasive scoring systems of this kind reliably exclude advanced fibrosis in MASLD ^[12], which is exactly what a primary-care triage tool needs to do: most people who screen negative can stop there. Non-invasive fibrosis markers have also been shown to associate with mortality in a US population sample ^[13], and the enhanced liver fibrosis panel and NAFLD fibrosis score have been compared against each other and against imaging ^[14].

The important caveat is that these scores are not equally reliable across ages: age is a confounder for the accurate non-invasive diagnosis of advanced fibrosis, and the standard thresholds misclassify older patients ^[15].

Elastography

Transient elastography (FibroScan, or vibration-controlled transient elastography) measures how fast a mechanical shear wave travels through the liver; stiffer, more fibrotic tissue transmits it faster, giving a liver stiffness measurement in kilopascals. Its performance for staging fibrosis has been meta-analysed ^[16], the technique and its clinical applications reviewed ^[17], and the approach was originally validated against biopsy in chronic viral hepatitis before moving into metabolic liver disease ^{[18] [19]}. It is quick, painless and repeatable, and it sits between the blood-based scores and biopsy in the diagnostic pathway. Magnetic resonance elastography performs better still, and laboratory tests, ultrasound and MR elastography have been compared head-to-head for detecting fibrosis in MASLD ^[14]. Practice guidance from the American Association for the Study of Liver Diseases sets out how these fit together ^{[9] [20] [21]}, as do the Asia-Pacific guidelines ^[22].

Centerpiece: a simple simulatable model of fibrosis progression

If fibrosis stage is what determines outcome, then the quantity that matters over a lifetime is how fast people climb the ladder — and what happens if you slow them down.

Model a cohort as a five-state Markov chain over F0…F4, with cirrhosis absorbing. Each year, a patient either stays at their stage or advances one:

P(Fᵢ → Fᵢ₊₁) = p per year, with F4 absorbing.

That leaves one parameter, and it does not have to be guessed. The same global meta-analysis that produced the prevalence figure also reported progression: among people with NASH, the mean annual rate of fibrosis progression was 0.09 stages per year (95% CI 0.06–0.12), with 40.76% showing progression at all ^[7]. A rate of 0.09 stages per year is one fibrosis stage roughly every eleven years.

What the model explains. Four things.

First, why this disease is invisible for so long. At 0.09 stages a year, a person who develops MASH in their thirties reaches advanced fibrosis in their fifties. Nothing hurts along the way. The model's early years are almost flat — which is exactly the period in which the disease is both most treatable and least likely to be noticed.

Second, why slowing progression is worth so much. Halving the transition probability takes the fraction with advanced fibrosis at 20 years from 27% to 6%. That is the entire therapeutic argument for treating a symptomless condition, and it does not require reversing anything — only slowing it.

Third, what the two independent sources say when combined. Weighting the model's stage distribution by the published stage-specific liver-related mortality ratios gives a cohort-average liver-related mortality ratio rising from 1.0 at baseline to about 10 at twenty years — and only about 4 if progression is halved. Neither paper computes that number; it falls out of putting a progression rate ^[7] and an outcome ladder ^[10] on the same axis.

Fourth, why "fatty liver" is a misleading name for the thing that matters. Steatosis is the entry ticket. The prognosis is set by where someone sits on the fibrosis ladder and how fast they are moving, which is why the endpoints in modern trials are fibrosis improvement and MASH resolution rather than liver fat content.

What the model deliberately does not do. It cannot regress. Real fibrosis improves in a substantial minority — that is precisely what the drugs in Pillar 2 are measured on — and a one-directional chain structurally cannot represent it, so the model overstates long-run accumulation. It uses one rate for everyone when progression varies enormously with diabetes, obesity and genotype. And it describes a MASH cohort, not the much larger population with simple steatosis, most of whom will never reach F3 at all.

Pillar 2: medicines and management

Weight loss is the foundation

Losing weight is the one intervention that acts on the cause. Sustained weight loss improves steatosis, inflammation and — at larger magnitudes — fibrosis, which is why lifestyle intervention remains first-line in every guideline, including the 2024 European guidelines written under the new nomenclature ^{[9] [20] [23]}. It is also difficult to sustain, which is why the pharmacological and surgical routes below have become central rather than supplementary. Bariatric and metabolic surgery produces the largest and most durable weight loss, and its indications were revised in a 2022 joint statement ^[24]; the liver lesions of patients undergoing it have been characterised histologically in their own right ^[25].

The metabolic comorbidities — and the cause of death

This is the single most under-appreciated fact about MASLD, and it changes what treatment means: most people with MASLD die of cardiovascular disease, not liver disease. The condition travels with obesity, type 2 diabetes, hypertension and dyslipidaemia ^[7], and treating those is not a distraction from liver care — for the majority of patients it is the intervention most likely to extend life. Liver-specific mortality in MASLD is elevated (incidence rate ratio 1.94) but the absolute liver-specific mortality rate, 0.77 per 1,000 person-years, is far below the overall mortality rate of 15.44 per 1,000 person-years ^[7]. The heart failure review covers the therapies that follow from that.

Drugs that act on the liver

Pioglitazone and vitamin E were the first agents with histological benefit in the landmark PIVENS trial, which tested both against placebo in non-diabetic patients with NASH ^[26]. Neither became a general solution — pioglitazone causes weight gain and fluid retention, vitamin E has its own safety questions — but the trial established that histology could be moved by a drug.

Resmetirom is the first approved MASH therapy. It is an oral, liver-directed, thyroid hormone receptor beta-selective agonist — the receptor subtype predominant in hepatocytes, which allows metabolic effects in the liver while largely avoiding the cardiac and bone effects of general thyroid hormone excess ^{[27] [28] [29]}. In the phase 3 MAESTRO-NASH trial, 966 patients with biopsy-confirmed NASH and fibrosis F1B–F3 received resmetirom 80 mg, 100 mg or placebo. Both primary endpoints were met at week 52: NASH resolution with no worsening of fibrosis in 25.9% (80 mg) and 29.9% (100 mg) versus 9.7% on placebo, and fibrosis improvement by at least one stage with no worsening of the activity score in 24.2% and 25.9% versus 14.2% ^[2]. LDL cholesterol also fell 13.6–16.3%. Diarrhoea and nausea were more frequent than with placebo; serious adverse events were similar across groups.

Read those numbers carefully. They are a genuine advance and the first of their kind — and they mean that roughly one patient in four improved a fibrosis stage against roughly one in seven on placebo. This is a real effect on a difficult endpoint, not a cure.

GLP-1 receptor agonists attack the problem from the metabolic side. In a 72-week phase 2 trial, once-daily subcutaneous semaglutide achieved NASH resolution with no worsening of fibrosis in 59% of the 0.4 mg group versus 17% on placebo — a large effect — with mean weight loss of 13% against 1%. But the confirmatory secondary endpoint failed: improvement in fibrosis stage occurred in 43% versus 33%, P = 0.48 ^[30]. Semaglutide has since been tested in MASH-related cirrhosis ^[31], and a meta-analysis of randomised trials of GLP-1 receptor agonists in MASH and fibrosis has been published ^[32]. The honest summary is that GLP-1 agents clearly resolve steatohepatitis and clearly produce weight loss; their effect on the fibrosis endpoint that the model above identifies as decisive is less firmly established.

Pillar 3: progress

Replacing the biopsy

The direction of travel is toward staging without a needle. Blood-based scores triage ^{[12] [13]}, elastography stages ^[14], and trials increasingly use non-invasive endpoints alongside histology — resmetirom's phase 2 extension study was reported specifically on non-invasive endpoints ^[27]. Given a disease affecting a quarter of adults, a pathway that reserves biopsy for the genuinely uncertain is not a convenience but a precondition for treating the condition at scale.

The pipeline

Resmetirom's approval opened a field rather than closing it. Thyroid hormone receptor beta agonism, GLP-1 and dual incretin agonism, and other targets in cholestatic and fatty liver disease are all in development ^{[29] [28]}. The dual GIP/GLP-1 co-agonist tirzepatide has been examined for its effect on liver fat content by MRI in type 2 diabetes ^{[33] [34]}. The field also has failures worth remembering: elafibranor, a PPAR-α/δ agonist, induced NASH resolution in phase 2 ^[35] and did not survive phase 3. The strategic question the model above frames is which endpoint each should be judged on: NASH resolution is easier to move and fibrosis improvement is what predicts survival.

Cancer, and the population problem

MASLD raises hepatocellular carcinoma risk — HCC incidence among NAFLD patients was 0.44 per 1,000 person-years ^[7] — and, unusually, it can do so without cirrhosis, which complicates surveillance because the usual trigger for screening is absent. Dietary cholesterol has been shown to drive fatty-liver-associated liver cancer through gut microbiota and metabolites in experimental work ^[36]. Against a background prevalence of a quarter of all adults, even a small per-person risk becomes a large number of cancers, and identifying who to watch is an open problem.