Fatty liver disease is a condition where excess fat builds up inside liver cells. About 1.3 billion people worldwide have it, making it one of the most common liver conditions on the planet. Most people with early-stage fatty liver disease feel perfectly fine, which is part of what makes it so easy to miss.
The condition exists on a spectrum. At one end, fat simply accumulates in the liver without causing much harm. At the other end, that fat triggers inflammation, scarring, and eventually serious liver damage. Understanding where you fall on that spectrum, and what drives progression, is the key to managing it.
The Name Has Changed
If you’ve seen the terms NAFLD or NASH before, those names were officially retired in 2023. The umbrella term is now steatotic liver disease (SLD), and the most common form, previously called NAFLD, is now called metabolic dysfunction-associated steatotic liver disease, or MASLD. The inflammatory stage formerly known as NASH is now MASH.
The renaming wasn’t just cosmetic. Doctors and patients pointed out that “fatty” in the old name carried stigma, and “nonalcoholic” defined the disease by what it wasn’t rather than what it was. The new names reflect what actually drives the condition: metabolic factors like insulin resistance, excess body weight, and abnormal blood lipids. A separate category called MetALD now covers people who have metabolic liver disease and also drink moderate to heavy amounts of alcohol.
How Fat Builds Up in the Liver
The liver normally processes and exports fat. Problems start when more fat flows into the liver than it can handle, or when the liver starts manufacturing too much fat on its own. Insulin resistance is the central driver. When your cells stop responding properly to insulin, fat that would normally be stored in fat tissue gets rerouted to the liver instead.
Once inside liver cells, certain fat molecules interfere with insulin signaling even further, creating a vicious cycle. Fat droplets accumulate in the cell, the liver becomes less responsive to insulin, and more fat keeps arriving. Research in humans has shown that the fat content inside liver cell droplets is the single best predictor of insulin resistance, accounting for roughly 64% of the variation in how well a person responds to insulin.
Diet plays a direct role too. Fructose is especially efficient at driving liver fat production because, unlike glucose, it’s processed almost entirely by the liver and bypasses the normal regulatory checkpoints that limit fat creation. In controlled studies, a high-fructose diet increased the liver’s rate of converting sugar to fat by about 70% compared to a diet with the same calories from complex carbohydrates. Liver fat itself rose by a median of 37%. On top of producing new fat, this process also blocks the liver from burning existing fat for energy, so the accumulation compounds.
Stages of Liver Damage
Fatty liver disease progresses through distinct stages, and most people never advance beyond the first one.
- Simple steatosis. Fat is present in the liver but there’s no significant inflammation or scarring. This is the most common stage and the least dangerous.
- Steatohepatitis (MASH). The fat has triggered inflammation, and liver cells are being damaged. This is the stage where the disease becomes more serious, because inflammation is what leads to scarring.
- Fibrosis. Repeated inflammation causes scar tissue to form. Fibrosis is graded from F0 (no scarring) through F4 (cirrhosis). Fibrosis stage is the single most important factor in determining long-term outcomes.
- Cirrhosis (F4). Extensive scarring has replaced healthy liver tissue, impairing the liver’s ability to function. At this point, the damage is largely irreversible and can lead to liver failure or liver cancer.
The progression from simple fat accumulation to cirrhosis typically takes years or decades, and many people with simple steatosis never develop significant scarring at all. But because the disease is silent for so long, some people are diagnosed only after substantial damage has already occurred.
Symptoms to Watch For
In its early stages, fatty liver disease usually causes no symptoms at all. When symptoms do appear, they tend to be vague: a feeling of fullness or discomfort in the upper right side of your abdomen, or persistent fatigue and weakness that doesn’t seem to have another explanation.
More noticeable signs typically only show up once the disease has progressed to advanced fibrosis or cirrhosis. These can include yellowing of the skin and eyes (jaundice), swelling in the legs or abdomen, easy bruising, and confusion. An enlarged liver may be detected during a routine physical exam before you notice anything yourself, which is one reason regular checkups matter.
How It’s Diagnosed
Fatty liver disease is often first suspected from routine blood work that shows elevated liver enzymes, or from an imaging scan done for another reason. But blood tests can be normal even when significant fat or scarring is present, so a normal result doesn’t rule it out.
A specialized ultrasound-based scan called FibroScan can measure both liver fat and stiffness in a single, painless office visit. It produces two scores. The CAP score measures fat content: below 238 dB/m is considered normal, 238 to 260 indicates mild fat accumulation (roughly 11 to 33% of the liver affected), 260 to 290 is moderate (34 to 66%), and above 290 means more than two-thirds of the liver contains excess fat. The stiffness score measures scarring: normal readings fall between 2 and 7 kPa, while a reading of 14 kPa or higher suggests cirrhosis.
In some cases, a liver biopsy is still needed to confirm how much inflammation and scarring is present, particularly when treatment decisions depend on the exact stage of fibrosis.
Who’s Most at Risk
The metabolic factors that drive fatty liver disease overlap heavily with those behind type 2 diabetes and heart disease. Carrying excess weight, particularly around the midsection, is the strongest risk factor. Insulin resistance, high triglycerides, high blood pressure, and elevated blood sugar all increase your likelihood. You don’t need to have all of these. Any one of them raises your risk.
Globally, about 16% of the population has the condition, but rates vary dramatically by region. North Africa and the Middle East have the highest prevalence, approaching 29%. Genetics play a role in susceptibility as well. Some people develop fatty liver disease at a relatively normal weight, while others with significant obesity never do.
A diet high in added sugars, particularly sugary drinks and processed foods rich in fructose, contributes independently of body weight. Sedentary behavior compounds the problem by reducing the muscles’ ability to take up glucose and fat from the bloodstream, leaving more for the liver to handle.
Treatment and Weight Loss Thresholds
Weight loss is the most effective treatment for fatty liver disease, and the amount you need to lose depends on what you’re trying to achieve. Losing 3 to 5% of your body weight is typically enough for fat to start clearing from the liver. For someone who weighs 200 pounds, that’s 6 to 10 pounds. But reversing inflammation and improving scarring requires more: at least 10% body weight loss.
The type of exercise matters less than consistency. Both aerobic activity and resistance training reduce liver fat, even when weight loss is modest. Cutting back on added sugars, especially fructose from sweetened beverages, directly reduces the liver’s fat-producing machinery.
For people who have already progressed to moderate or advanced fibrosis (stages F2 or F3) without cirrhosis, there is now a medication option. In 2024, the FDA approved the first drug specifically for this stage of the disease. It works by activating a thyroid hormone receptor in the liver that helps clear fat and reduce inflammation. It’s prescribed alongside diet and exercise, not as a replacement, and is not intended for people who have already developed cirrhosis.
For people with obesity who struggle to lose weight through lifestyle changes alone, weight-loss medications or bariatric surgery can produce the degree of weight loss needed to meaningfully improve liver health. The liver’s ability to recover is remarkably strong in the earlier stages. Even moderate fibrosis can improve or stabilize if the underlying metabolic drivers are addressed.