A fatty liver is a condition where excess fat builds up inside liver cells. A healthy liver contains a small amount of fat, but when fat exceeds about 5% of the liver’s weight, it’s classified as fatty liver disease. Roughly 1.3 billion people worldwide, about 16% of the global population, are living with it as of 2023.
How Fat Builds Up in the Liver
Your liver processes the fats and sugars you eat, either burning them for energy or packaging them into particles that get shipped out to the rest of your body. Fatty liver develops when energy from food enters the liver faster than the liver can burn it off or export it. The surplus gets stored as fat droplets inside liver cells called hepatocytes.
Fructose, a sugar found in sweetened drinks and many processed foods, plays an outsized role. Unlike other nutrients that get distributed across the body, fructose is funneled almost exclusively to the liver, where it gets converted directly into fat through a process called de novo lipogenesis. This is one reason sugary drinks are so strongly linked to liver fat accumulation, even in people who aren’t significantly overweight.
The Name Has Changed
If you’ve seen the term NAFLD (nonalcoholic fatty liver disease) before, that name has been officially retired. The condition is now called MASLD, which stands for metabolic dysfunction-associated steatotic liver disease. The more severe inflammatory form, previously known as NASH, is now called MASH (metabolic dysfunction-associated steatohepatitis). The broader umbrella term for any type of liver fat, regardless of cause, is steatotic liver disease.
The name change matters because the old label defined the condition by what it wasn’t (not caused by alcohol) rather than what it was. MASLD is defined by what’s actually driving the disease: metabolic dysfunction. To qualify for a MASLD diagnosis, you need liver fat plus at least one of five cardiometabolic risk factors: being overweight or obese, elevated blood sugar or type 2 diabetes, high blood pressure, high triglycerides, or low HDL cholesterol.
There’s also a newer in-between category called MetALD, for people who have metabolic risk factors and drink moderate-to-heavy amounts of alcohol (roughly 1.5 to 4 drinks per day for women, or 2 to 6 for men). This acknowledges that metabolism and alcohol often overlap as causes.
Why Most People Don’t Know They Have It
In its early stages, fatty liver causes no symptoms at all. Most people find out about it incidentally, during an imaging scan or blood test done for another reason. This is part of what makes the condition so widespread: it can silently progress for years.
When symptoms do eventually appear, they tend to be vague. Fatigue and a dull ache or feeling of fullness in the upper right side of the abdomen are the most common complaints. More noticeable signs like yellowing of the skin and eyes, swelling in the legs and belly, or easy bruising only show up once the liver is significantly damaged, typically at the cirrhosis stage.
What Causes It
Fatty liver is tightly linked to the same cluster of conditions that raise your risk for heart disease and diabetes. The most common risk factors include:
- Excess weight, particularly fat stored around the waist
- Insulin resistance, where cells stop responding normally to insulin, forcing the body to produce more
- Type 2 diabetes
- High triglycerides or low HDL cholesterol
- High blood pressure
Other conditions that increase risk include polycystic ovary syndrome and obstructive sleep apnea. Genetics also play a role. Some people accumulate liver fat more readily than others at the same body weight, and certain genetic variants significantly increase susceptibility.
Insulin resistance is the central driver for most people. When your cells resist insulin’s signal to absorb sugar from the blood, the liver compensates by converting that excess sugar into fat. Over time, this creates a feedback loop: liver fat worsens insulin resistance, which in turn drives more fat storage.
How It Progresses
Not everyone with a fatty liver gets sicker. Many people live with simple fat accumulation (steatosis) and never develop serious liver problems. But in a subset of people, the fat triggers inflammation and damage to liver cells, a condition now called MASH. In MASH, liver cells become injured and swollen, a process pathologists call “ballooning.” These damaged cells send out inflammatory and scarring signals to surrounding tissue.
Doctors track progression using a fibrosis scoring system from F0 to F4:
- F0: No scarring
- F1: Mild scarring
- F2: Moderate scarring
- F3: Advanced scarring
- F4: Cirrhosis
Fibrosis stage is the single most important factor in determining outcomes. At F4 (cirrhosis), the liver is so heavily scarred that it begins to lose function. Cirrhosis can lead to liver failure or liver cancer. The progression from simple fatty liver to cirrhosis typically takes many years, sometimes decades, and the earlier stages are reversible with the right interventions.
How It’s Diagnosed
Fatty liver is often first suspected when routine blood work shows mildly elevated liver enzymes. But blood tests alone can’t confirm it or tell you how much fat or scarring is present.
A standard abdominal ultrasound can detect fat in the liver, though it’s not very precise at measuring the amount. For a more detailed picture, many doctors now use a specialized ultrasound called FibroScan (liver elastography). This painless test takes about 10 minutes and measures two things simultaneously. First, it calculates a CAP score, which estimates the percentage of fat in your liver. Scores range from 100 to 400 decibels per meter, with anything below 238 considered normal. Second, it measures liver stiffness in kilopascals. Normal readings fall between 2 and 7 kPa, and higher numbers suggest more scarring. The highest possible reading is 75 kPa.
FibroScan results can be less accurate in people with obesity (BMI over 30), fluid buildup in the abdomen, or certain other liver conditions including heart failure-related congestion. Liver stiffness readings can also be temporarily elevated by active inflammation, which doesn’t necessarily mean permanent scarring.
Weight Loss Is the Primary Treatment
No widely available medication specifically treats fatty liver yet, making lifestyle changes the cornerstone of management. Weight loss is the most effective intervention, and the target 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 liver cells. That’s about 6 to 10 pounds for someone who weighs 200 pounds. But if inflammation and scarring are already present, a weight loss of around 10% is needed to improve them.
The Mediterranean diet has the strongest evidence behind it. In clinical studies, this eating pattern, built around vegetables, whole grains, olive oil, fish, and legumes while limiting sugar and saturated fat, has been shown to both prevent and improve fatty liver disease. One well-studied version of the diet uses a rough ratio of 40% carbohydrates, 40% fat (mostly from olive oil and fish), and 20% protein, with an emphasis on low-glycemic foods that don’t spike blood sugar.
Calorie restriction also works, with study protocols typically using about 1,500 calories per day for women and 1,800 for men. But the type of calories matters nearly as much as the total. Cutting back on added sugars, especially fructose from sweetened beverages and processed foods, directly reduces the raw material the liver uses to make fat.
Exercise Helps Even Without Weight Loss
Regular physical activity reduces liver fat independently of whether the number on the scale changes. Both aerobic exercise (walking, cycling, swimming) and resistance training (weight lifting) have been shown to lower liver fat. This is particularly important because it means that even people who struggle to lose weight can still improve their liver health by staying active. Most guidelines suggest at least 150 minutes of moderate-intensity activity per week as a starting target.
Alcohol deserves a mention even though MASLD isn’t caused by drinking. If you already have liver fat from metabolic causes, alcohol adds a second source of stress to an already burdened organ. The MetALD category exists precisely because many people with metabolic fatty liver also drink enough to compound the damage. Reducing or eliminating alcohol can meaningfully slow progression.
The Heart Risk Matters Too
People with fatty liver are more likely to die from heart disease than from liver disease itself. Because the same metabolic dysfunction that drives fat into the liver also damages blood vessels and raises cardiovascular risk, managing blood pressure, blood sugar, and cholesterol is just as important as addressing the liver directly. Treating fatty liver isn’t only about protecting your liver. It’s about addressing the broader metabolic picture that threatens your heart, kidneys, and blood vessels at the same time.