What is fatty liver disease (hepatic steatosis)?

For a long time, fatty liver disease was associated almost exclusively with excessive alcohol consumption. Today, in its metabolic form, it affects more than a billion people worldwide, most of them with no connection to alcohol at all.

The expression “fatty liver” might sound almost comical, but it describes exactly what happens. As with a force-fed goose, the liver's cells become loaded with droplets of fat. In humans, however, it isn't force-feeding that causes this, but a sustained energy surplus relative to the body's needs, occurring in a context where the body manages sugar and fat poorly.

A buildup of fat in the liver

Fatty liver disease corresponds to an excessive buildup of fat within liver cells. By convention, the term steatosis is used once more than 5% of liver cells contain fat droplets.

Since 2023, an international scientific consensus has revised this terminology. What used to be called NAFLD (non-alcoholic fatty liver disease) is now referred to as MASLD (Metabolic dysfunction-Associated Steatotic Liver Disease) — that is, steatotic liver disease associated with metabolic dysfunction. This change reflects a better understanding of the condition: its main cause is not the absence of alcohol, but rather the presence of one or more metabolic risk factors. These factors are type 2 diabetes, overweight or obesity, high blood pressure, and abnormal cholesterol levels.

This consensus, published by the leading European, American, and Latin American liver disease societies, was developed with input from more than 200 experts as well as patient associations. The old term had two shortcomings: it defined the disease by what it was not, and the word “alcoholic” felt stigmatizing to many patients who did not drink at all. The new classification also introduces an intermediate category, MetALD, for people who combine metabolic risk factors with moderate-to-heavy alcohol consumption — a situation that is common in real life, where the two causes add up to one another.

Why does the liver store fat in the first place? When the body's cells become less sensitive to insulin — a state called insulin resistance — fat tissue releases more fatty acids into the bloodstream, and the liver absorbs them. At the same time, an excess of sugars, fructose in particular, stimulates fat production directly inside the liver. When what comes in exceeds what the liver is able to burn off or export, fat accumulates. Deep abdominal fat, the kind that surrounds the internal organs, plays a particular role here, since it drains directly into the liver. This is why waist circumference is often more informative than weight on its own.

A very high prevalence, everywhere in the world

In France, according to the CONSTANCES cohort, about 16.7% of the adult population is affected. The difference between the sexes is clear: 24.6% of men compared with 10.1% of women. Prevalence rises above 36% among men aged 68 to 78. These figures rely on an index calculated from waist circumference, BMI, and blood markers, rather than on an ultrasound scan. They should therefore be read as an order of magnitude rather than as an exact measurement.

In the United States, prevalence is even higher: about one adult in three is thought to be affected, and roughly 5% of adults are estimated to have the inflammatory form, steatohepatitis. Studies using more recent imaging techniques find figures that come even closer to 40%.

At the global level, the latest Global Burden of Disease analysis, published in The Lancet Gastroenterology & Hepatology, estimates that around 1.3 billion people were living with MASLD in 2023 — about 16% of the world's population, children included. That represents a 143% increase since 1990, and projections point to around 1.8 billion people by 2050. Among adults alone, imaging-based meta-analyses place prevalence at around 30%, or nearly one adult in three. The regions most affected are the Middle East, North Africa, and Latin America. This rise closely tracks the rise in obesity and diabetes.

A condition that's usually mild, but not always

In most cases — about 80% according to estimates — steatosis remains isolated and benign, without serious consequences. In the remaining cases, it progresses to liver inflammation known as steatohepatitis. Its abbreviation is MASH, for Metabolic dysfunction-Associated SteatoHepatitis — that is, steatohepatitis associated with metabolic dysfunction. It used to be called NASH (Non-Alcoholic SteatoHepatitis). This more serious stage can eventually progress to fibrosis, cirrhosis, or even liver cancer in the most advanced forms.

In steatohepatitis, the excess fat becomes toxic to liver cells: they become inflamed, suffer damage, and die, and the liver attempts to repair this damage by producing scar tissue — this is fibrosis. Fibrosis is classified into stages, ranging from F0 (no fibrosis) to F4 (cirrhosis). This progression is generally slow: follow-up studies show that it takes several years on average to move up a single stage, with large differences from one person to another.

Several factors help explain why some people progress faster than others. The most important is type 2 diabetes, which roughly doubles the risk of progressing to advanced fibrosis. Age beyond 50 is another factor, as is the accumulation of several metabolic abnormalities at once. Alcohol consumption, even at moderate levels, adds to the metabolic injury already under way. Smoking is also associated with more severe fibrosis. Finally, certain genetic predispositions speed up the disease's progression. Conversely, weight loss or better control of diabetes can slow this progression down, or even reverse it.

The stage of fibrosis is the main factor that determines the outlook for people affected by the disease. Steatosis without significant fibrosis carries a prognosis close to that of the general population. Advanced fibrosis, by contrast, significantly increases the risk of liver-related complications. This is why screening tools, such as the FIB-4 score, aim above all to detect fibrosis itself, rather than simply the presence of fat. MASLD has also become one of the fastest-rising causes of liver cancer, and this cancer can sometimes appear even before the cirrhosis stage is reached.

A close link with diabetes and obesity

In people with type 2 diabetes, fatty liver disease is two to three times more common than in the general population: it affects more than one diabetic person in two, and it also progresses faster toward severe forms in this group. Steatosis also affects close to 80% of people living with obesity. This overlap is not a coincidence — it reflects a shared metabolic origin.

This relationship actually runs in both directions. A liver already loaded with fat itself becomes resistant to insulin and releases too much glucose into the blood, which increases the risk of developing diabetes in the years that follow. The hub's Am I at risk of diabetes? test can help estimate this risk.

It's also worth knowing that a lean person can have fatty liver disease. About 10% to 20% of affected people have a normal body weight, particularly in Asia. These are often people with little muscle mass, fat concentrated mainly in the abdomen, or a genetic predisposition. A variant of a gene called PNPLA3, for instance, which is common in certain populations, promotes fat buildup in the liver independently of body weight. Waist circumference, as assessed by the Am I overweight/obese? tool, is a better indicator than BMI on its own.

Finally, the disease also affects overweight children and teenagers. It is now the most common chronic liver disease among children in Western countries.

To estimate your own risk level, take the “Do I have fatty liver disease?” test.