What is fatty liver?

What fatty liver means

Fatty liver is the everyday name for hepatic steatosis, a condition in which an excess of fat is stored inside the cells of the liver. A small amount of fat in the liver is normal. Clinicians generally use the term steatosis when that fat exceeds a threshold — often described as roughly five percent of the liver's weight, or a similar proportion of liver cells visibly holding fat droplets. This article describes that biology at a general level and is educational only; it is not medical advice and does not diagnose or describe treatment for any condition.

The liver is a central hub for processing fats, sugars, and many other substances the body handles. Because of that role, the amount of fat it holds reflects the balance of several metabolic pathways at once. Fatty liver is broadly separated into two contexts: a form associated with alcohol, and a form associated with metabolic factors that is unrelated to heavy alcohol use. The sections below focus mainly on the metabolic context and on the cellular biology of how fat comes to accumulate.

How fat builds up inside liver cells

Under normal conditions the liver constantly takes in, uses, and sends out fat. It receives fatty acids from the bloodstream, burns some of them for energy through a process called fatty-acid oxidation, and packages others into particles called VLDL that carry fat back out to the rest of the body. It can also store fat internally as triglyceride, held in microscopic compartments called lipid droplets. Steatosis is best understood as an imbalance in this traffic — a state in which more triglyceride is entering or being built inside the liver cell than is being exported or burned. Researchers generally describe the fat within a steatotic liver as arriving from a few distinct sources:

• Fatty acids released from the body's fat tissue and delivered to the liver through the blood.
• Fat from the diet, carried to the liver as the remnants of the particles that transport dietary fat.
• New fatty acids the liver builds itself from excess sugar, through a process called de novo lipogenesis.

When the combined inflow from these sources outpaces the liver's capacity to export and oxidize fat, triglyceride accumulates inside hepatocytes as lipid droplets — the hallmark of steatosis. Framed this way, fatty liver is less a single event than the visible result of several metabolic flows tilting out of balance.

The metabolic context: insulin and lipogenesis

In the metabolic form of fatty liver, insulin resistance is a recurring feature of the surrounding biology. Insulin normally signals fat tissue to hold on to its stored fat and signals the liver to limit its own glucose output. When tissues respond less readily to insulin, fat tissue tends to release more free fatty acids into the bloodstream, and a larger share of those fatty acids is delivered to the liver.

At the same time, the body often circulates higher levels of insulin in the setting of insulin resistance. Insulin is one of the signals that switches on de novo lipogenesis — the liver's own fatty-acid manufacturing — through transcription factors such as SREBP-1c, while dietary sugars, particularly fructose, are associated with a related factor called ChREBP. The picture researchers describe is one in which the liver is receiving more fatty acids and, at the same time, building more of its own. These are descriptions of how the pathways are understood to interact, not statements about any individual person.

From stored fat to cellular stress

Not all fatty liver looks the same under a microscope. Researchers describe a spectrum. At one end is simple steatosis, in which liver cells hold excess fat but show little sign of injury or inflammation. Further along the spectrum, the picture can include inflammation and signs of cell stress — a state referred to as steatohepatitis — and, in some cases, the gradual buildup of scar tissue known as fibrosis.

The biology scientists study to explain this difference is often grouped under the term lipotoxicity. The idea is that it may not be simply the quantity of fat but the specific types of lipid molecules — and how the cell handles them — that are associated with stress inside the cell. Certain lipid species have been linked in research to stress in the endoplasmic reticulum and mitochondria, to the generation of reactive oxygen species, and to inflammatory signaling. Whether and how a given person moves along this spectrum varies widely; that is a clinical question for a licensed provider, and this article does not describe how to change its course.

How fatty liver is named and identified

The terminology for fatty liver has shifted over time. The metabolic form was long called non-alcoholic fatty liver disease, or NAFLD, with its inflammatory form termed non-alcoholic steatohepatitis, or NASH. In 2023, international groups introduced updated names that center the metabolic context: steatotic liver disease as an umbrella term, with metabolic dysfunction-associated steatotic liver disease (MASLD) and its inflammatory form (MASH) describing the metabolic category. The alcohol-associated form is classified separately.

Fatty liver is often first noticed incidentally — for example, through imaging performed for another reason, or through blood tests that reflect liver activity. Because none of those findings interpret themselves, determining whether excess fat is present, where a person sits on the spectrum, and what it means for them is a matter for a qualified clinician rather than something to conclude from a general explainer.

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This article is educational only and is not a substitute for a conversation with your own healthcare provider.