Liver fibrosis vs. fatty liver: what's the difference?

“Fatty liver,” “steatosis,” “fibrosis,” “cirrhosis”: these words often come up together on an ultrasound report or in a doctor's explanation, and it's easy to mix them up. Yet they describe different things. Steatosis is a build-up of fat in the liver. Fibrosis is a scar. You can have one without the other, and it's the second that mainly determines the liver's future. This article explains both concepts, then compares them point by point.

Steatosis: fat in the liver

Liver steatosis, commonly called “fatty liver,” is an excess of fat stored in liver cells. By convention, steatosis is diagnosed when more than 5% of liver cells contain fat droplets.

Its most common causes are excess weight, especially when fat settles around the waist, type 2 diabetes, high triglycerides, and alcohol use. When it's linked to these metabolic factors, it's called MASLD (Metabolic dysfunction-Associated Steatotic Liver Disease). This is the new name for what used to be called NAFLD.

Steatosis is very common. In France, analyses from the CONSTANCES cohort estimate that it affects 17 to 18% of adults, or nearly one in five people. In the United States, about one in three adults would be affected. Worldwide, the latest Global Burden of Disease analysis estimates that 1.3 billion people were living with MASLD in 2023.

In the vast majority of cases, steatosis stays isolated: the liver is loaded with fat, but it isn't inflamed or damaged. At this stage, it causes no symptoms and is reversible. A few months of lifestyle changes are often enough to significantly reduce it.

Fibrosis: a scar in the liver

Fibrosis is a different kind of process. It isn't a deposit, but the liver's response to an ongoing injury. Each time liver cells are repeatedly damaged — by alcohol, a virus, fat that has become toxic, or another cause — the organ repairs itself by producing scar tissue rich in collagen. It's the same mechanism as the scar that forms on skin after a wound.

A small scar doesn't get in the way. But if the injury continues for years, the scars build up, thicken, and eventually connect different areas of the liver. Normal, supple tissue is gradually replaced by hard tissue that no longer performs the liver's functions and that hampers blood flow inside the organ.

Doctors classify fibrosis into five stages, using the METAVIR score:

• F0: no fibrosis
• F1: mild fibrosis
• F2: moderate fibrosis, called “significant”
• F3: severe fibrosis, called “advanced”
• F4: cirrhosis

Cirrhosis isn't a separate disease: it's the final stage of fibrosis, where scarring has disorganized the architecture of the entire liver. It can have many causes, not just alcohol, contrary to a still-common belief.

How does one lead to the other?

Steatosis is one of the gateways to fibrosis, but this progression is far from automatic. According to the French national society of gastroenterology (SNFGE), about 80% of people with steatosis keep a simple form. In the remaining 20%, the fat eventually inflames the liver: this is steatohepatitis, or MASH (Metabolic dysfunction-Associated SteatoHepatitis). It's this inflammation, not the fat alone, that triggers scar formation. Overall, fewer than 5% of people with steatosis progress to advanced fibrosis, and about 1% to cirrhosis.

This progression is slow. A synthesis of studies with repeated biopsies, published in 2015 in Clinical Gastroenterology and Hepatology (Singh et al.), estimated that it took an average of 14 years to progress one fibrosis stage in simple steatosis, and about 7 years in steatohepatitis. These averages hide large differences between people: type 2 diabetes, age, alcohol, and certain genetic predispositions speed up progression.

The reverse is also true: fibrosis can exist without steatosis. Chronic hepatitis B or C, prolonged alcohol use, or a disease such as hemochromatosis can create scarring in a liver that isn't fatty.

Both, at a glance

Why fibrosis matters more

The amount of fat in the liver tells us little about the future. The fibrosis stage is the best indicator of prognosis. A 2021 American study published in the New England Journal of Medicine (Sanyal et al.) followed 1,773 adults with biopsy-confirmed steatosis for an average of four years. Mortality was 0.32 deaths per 100 person-years at stages F0 to F2, 0.89 at stage F3, and 1.76 at stage F4. Serious liver complications were nearly nonexistent before stage F3.

In other words, steatosis without significant fibrosis has a liver-related prognosis close to that of the general population, while advanced fibrosis warrants specialized follow-up. This is why the 2024 European guidelines (EASL-EASD-EASO) call for screening for fibrosis in people who have steatosis, type 2 diabetes, or obesity with other risk factors.

How do you know where you stand?

Neither steatosis nor fibrosis can be felt. Normal transaminases don't rule them out either. Both are detected with different, complementary tools.

For steatosis, the reference test in routine practice is ultrasound. Before that, an index such as the FLI (Fatty Liver Index) estimates the probability of fatty liver from waist circumference, BMI, triglycerides, and GGT. That's the purpose of the Do I have fatty liver disease? tool.

For fibrosis, the recommended first step is FIB-4 (Fibrosis-4 index). It's calculated from age, AST and ALT transaminases, and platelet count — four values found on a routine blood panel. A result below 1.30 makes advanced fibrosis unlikely. A result above 2.67 indicates high risk. In between, the result isn't conclusive, and a second test is offered: elastography, the best known being FibroScan, which measures liver stiffness in about ten minutes, painlessly. From age 65, the lower threshold is raised to 2.0, since the score naturally rises with age.

FIB-4 is a triage tool, not a diagnosis. It's mainly useful for reassurance: when it's low, it reliably rules out advanced fibrosis. When it's high, it doesn't prove the liver is damaged, but it indicates that further checking is needed.

What to remember

Steatosis is fat, fibrosis is a scar. The first is very common and usually benign; the second is rarer, but it's the one that leads to complications. Both are silent, and both can regress when the underlying injury decreases, all the more easily when caught early. A worrying result isn't a sentence: it's a reason to talk to your doctor, who has simple tests to clarify the situation.

To estimate your fibrosis risk from a blood test, take the “Is my liver damaged?” test.