Fatty liver develops when triglycerides, the main type of fat in your body, build up inside liver cells faster than the liver can process or export them. About 1.3 billion people worldwide live with this condition, making it the most common chronic liver disease on the planet. The causes range from what you eat and drink to your genetics, medications, and metabolic health.
How Fat Accumulates in the Liver
Your liver normally handles fat efficiently. It packages triglycerides into particles that get shipped out into the bloodstream, or it burns fat for energy. Fatty liver happens when this system gets overwhelmed. Too much fat flows in, too much new fat gets made inside the liver, or the liver’s ability to export and burn fat slows down.
Inside liver cells, fat gets stored in structures called lipid droplets. These droplets have a protective outer shell made of specific fats and proteins that control how triglycerides move in and out. When that shell loses integrity, cholesterol can leak from the droplets into other parts of the cell, particularly into compartments called lysosomes that act as the cell’s recycling centers. This damages the lysosomal membranes and can trigger cell death, which is how simple fat accumulation progresses to liver inflammation and more serious disease.
Metabolic Causes Are the Most Common
The vast majority of fatty liver cases are tied to metabolic dysfunction, meaning the body’s systems for processing sugar, fat, and energy aren’t working properly. The condition was recently renamed from “nonalcoholic fatty liver disease” (NAFLD) to “metabolic dysfunction-associated steatotic liver disease” (MASLD) to better reflect this connection. You only need one of the following metabolic risk factors alongside liver fat to qualify for the diagnosis:
- Overweight or obesity: a BMI of 25 or higher (23 in Asian populations), or a waist circumference above 94 cm for men or 80 cm for women
- Elevated blood sugar: fasting glucose of 100 mg/dL or above, an HbA1c of 5.7% or higher, or type 2 diabetes
- High blood pressure: 130/85 mmHg or above
- High triglycerides: 150 mg/dL or above
- Low HDL cholesterol: below 40 mg/dL for men or below 50 mg/dL for women
These factors cluster together. Insulin resistance is the thread connecting most of them. When your cells stop responding normally to insulin, your body stores more fat in the liver and ramps up production of new fat from sugar. The prevalence is consistently higher in men than women and varies significantly by region: North Africa and the Middle East have the highest rates, while high-income Asia Pacific countries have the lowest.
Sugar, Especially Fructose
Diet plays a direct role in liver fat accumulation, and fructose deserves special attention. Unlike glucose, which triggers insulin release that helps regulate how fast your body processes it, fructose has no immediate feedback mechanism to slow its own absorption or metabolism. Your liver converts fructose into an intermediate compound through an unrestricted process, meaning there’s no natural brake on how fast it happens.
This matters because fructose activates the liver’s fat-making machinery, a process called de novo lipogenesis, through multiple pathways simultaneously. At the same time, fructose exposure turns down the genes responsible for burning fat. So the liver is making more fat while burning less of it. Fructose also provides the raw carbon building blocks for assembling new triglycerides. The net result is a liver awash in freshly manufactured fat with fewer ways to get rid of it.
The biggest dietary sources of fructose are sugar-sweetened beverages, fruit juices, and processed foods with added sugars. Table sugar is half fructose, and high-fructose corn syrup is roughly the same. Whole fruit contains fructose too, but the fiber slows absorption enough that it doesn’t produce the same liver effects at normal intake levels.
Alcohol and Liver Fat
Alcohol is the other major dietary driver. When your liver breaks down ethanol, it produces a chemical byproduct that shifts the balance of a critical molecule called NAD+. Your liver needs NAD+ to burn fatty acids. Alcohol metabolism consumes NAD+ and converts it to a form the liver can’t use for fat burning, effectively shutting down the liver’s ability to process fat. The fat that would normally be burned for energy instead piles up inside liver cells.
This process begins quickly. Even moderate drinking over time can contribute to fat buildup. Heavy drinking accelerates it dramatically. The new naming system recognizes a category called MetALD for people who have both metabolic risk factors and significant alcohol intake, acknowledging that these two causes frequently overlap and compound each other.
Genetics and Ethnicity
Your genes can make you significantly more or less vulnerable to fatty liver, regardless of your lifestyle. The most well-studied genetic factor is a variant in the PNPLA3 gene. People who carry two copies of this variant (called I148M) have more than double the liver fat content compared to people with no copies. In a study of Hispanic children and adolescents, those with two copies of the variant had liver fat of about 11%, compared to roughly 5% in those with none, and 64% of them met the threshold for fatty liver disease.
This variant is most common in Hispanic populations, where about 48% carry at least one copy, compared to around 20% in other groups. That partly explains why fatty liver rates are highest in Hispanics and lowest in African Americans, a pattern seen in both children and adults. The PNPLA3 variant works by interfering with a protein that helps break down fat inside liver cells, essentially trapping triglycerides in lipid droplets where they continue to accumulate. Notably, this genetic effect shows up even in young children aged 8 to 10, well before other metabolic risk factors typically appear.
Medications That Promote Liver Fat
Several common medications can cause or worsen fatty liver as a side effect. The most notable include corticosteroids, which shift your metabolism toward fat storage; methotrexate, used for autoimmune conditions and some cancers; amiodarone, a heart rhythm medication; and tamoxifen, used in breast cancer treatment. Anabolic steroids, certain anti-seizure drugs, tetracycline antibiotics, and even some birth control pills can contribute. If you’re taking any of these long-term and develop elevated liver enzymes or other signs of liver trouble, the medication may be a factor worth discussing with your prescriber.
Rapid Weight Loss
This one surprises most people: losing weight too fast can actually cause fatty liver or make existing fatty liver worse. When you drastically cut calories, your body rapidly mobilizes fat from storage sites throughout the body. That fat floods into the bloodstream and overwhelms the liver’s processing capacity. Losing more than about 3.5 pounds per week has been shown to worsen liver fat accumulation and inflammation. This is one reason bariatric surgery patients are monitored closely in the early postoperative period, when weight loss is fastest. A steadier rate of weight loss, at or below 3.5 pounds per week, reduces liver fat and improves inflammation rather than worsening it.
How These Causes Overlap
Fatty liver rarely comes down to a single cause. Someone with the PNPLA3 gene variant who also drinks moderately and eats a high-sugar diet faces a compounding risk that’s greater than any one factor alone. Insulin resistance drives fat into the liver while fructose triggers the liver to manufacture even more. Alcohol shuts down the liver’s ability to burn the fat that’s already there. Genetics determine how efficiently your liver can clear fat from its cells in the first place.
Understanding which causes apply to you matters because the solutions differ. Cutting back on added sugars targets the fructose-driven fat production pathway. Reducing alcohol restores the liver’s fat-burning capacity. Losing weight gradually, rather than through crash dieting, gives the liver time to process mobilized fat without becoming overwhelmed. And for the genetic component, being aware of your higher baseline risk can motivate earlier attention to the factors you can control.