Fatty liver develops when excess fat builds up inside liver cells, and the causes range from how your body handles insulin to what you eat and drink. About 1.3 billion people worldwide, roughly 16% of the global population, are living with the condition as of 2023. The two broad categories are alcohol-related fatty liver and metabolic fatty liver (previously called non-alcoholic fatty liver disease, or NAFLD, now renamed MASLD). Most cases trace back to a handful of overlapping triggers.
Insulin Resistance Is the Central Driver
The single most important factor behind metabolic fatty liver is insulin resistance, a state where your cells stop responding normally to insulin. Insulin’s job includes telling fat cells to hold onto their stored fat. When that signal weakens, fat cells release a flood of fatty acids into the bloodstream, and the liver absorbs them.
At the same time, high insulin levels (the body’s attempt to compensate) flip on your liver’s internal fat-production machinery. The liver starts converting sugars into new fat at an accelerated rate, a process called de novo lipogenesis. So fat pours into the liver from two directions at once: from fat tissue breaking down too quickly and from the liver manufacturing its own fat.
To make things worse, insulin resistance also shuts down the liver’s ability to burn fat for energy. A key protein that normally promotes fat burning stays inactive when insulin levels are chronically high. The liver can’t export the excess fat efficiently either. The result is a liver that accumulates fat it can neither burn nor ship out. This creates a vicious cycle: the buildup of fatty acids inside liver cells further damages insulin signaling, which drives even more fat accumulation.
Diet, Especially Fructose and Added Sugars
What you eat directly feeds fatty liver, and fructose is a particularly potent contributor. Unlike glucose, which gets used throughout the body, fructose is processed almost entirely by the liver. Research shows fructose is a more powerful stimulator of liver fat production than glucose, largely because the liver ramps up the enzymes needed to convert fructose into fat.
This matters because the modern Western diet is saturated with added sugars, including fructose from table sugar and high-fructose corn syrup in sweetened drinks, packaged foods, and snacks. The excess fat production triggered by fructose doesn’t just fill up the liver. It also impairs insulin sensitivity and disrupts normal metabolism in ways that compound the problem over time. Diets high in refined carbohydrates and total calories contribute through the same general pathway: more raw material for the liver to convert into stored fat.
How Alcohol Causes Fatty Liver
Alcohol-related fatty liver follows a different but overlapping biochemical path. When your liver breaks down ethanol, it disrupts the normal balance of fat metabolism in several ways at once. Alcohol switches off a master energy sensor in liver cells (AMPK), which has two consequences: it blocks fat burning and simultaneously activates fat production.
The damage goes further. Alcohol metabolism generates toxic byproducts that harm mitochondria, the parts of your cells responsible for burning fat as fuel. With fat burning impaired and fat production ramped up, the liver also loses its ability to package and export fat into the bloodstream via transport particles. Alcohol even causes fat cells elsewhere in the body to release more fatty acids into circulation, adding to the liver’s burden. Essentially, alcohol attacks every exit route for fat while opening every entrance.
Obesity and Type 2 Diabetes
Excess body weight, particularly visceral fat stored around the organs, is strongly linked to fatty liver because it promotes the insulin resistance described above. Visceral fat is metabolically active, releasing fatty acids and inflammatory signals that travel directly to the liver through the portal vein.
The overlap with type 2 diabetes is striking. A pooled analysis of over 1.8 million patients found that roughly 65% of people with type 2 diabetes also have fatty liver disease, and about 32% have the more advanced inflammatory form. This connection runs both ways: fatty liver worsens blood sugar control, and poor blood sugar control accelerates liver fat buildup. The two conditions reinforce each other, which is why fatty liver is now formally defined in relation to metabolic risk factors like elevated blood sugar, high blood pressure, and abnormal cholesterol levels.
Genetics Play a Significant Role
Not everyone with the same diet and weight develops fatty liver, and genetics help explain why. The most well-studied genetic variant involves the PNPLA3 gene, which provides instructions for an enzyme that breaks down fat in liver and fat cells. A specific variant of this gene (called I148M) produces an enzyme with reduced activity, leading to fat buildup in the liver even in people who might otherwise be at lower risk.
Carrying this variant doesn’t just raise the odds of developing fatty liver. It also increases the risk of progression to liver inflammation, scarring, and even liver cancer. The same genetic variant worsens liver damage in people with hepatitis C or alcohol use disorder, suggesting it amplifies harm regardless of the original cause. Other genetic variants affecting fat metabolism and liver inflammation have been identified as well, but the PNPLA3 variant remains the most significant one discovered so far.
The Gut-Liver Connection
Your intestines and liver are directly connected by a major blood vessel, the portal vein, meaning everything your gut absorbs passes through the liver first. When the balance of gut bacteria shifts in unhealthy directions, it can damage the intestinal lining and make it more permeable. Bacterial fragments, particularly a compound called lipopolysaccharide, then leak through into portal blood and reach the liver, where they trigger inflammation and promote fat accumulation.
Gut bacteria also influence fatty liver through the substances they produce. Certain bacterial metabolites help keep liver fat production in check, while others stimulate it. A shortage of beneficial bacteria that produce butyrate, a compound that suppresses liver fat synthesis, tilts the balance toward fat accumulation. Gut bacteria also modify bile acids, which normally help regulate fat and sugar metabolism in the liver. When that bile acid signaling is disrupted, the liver loses another brake on fat production.
Choline, a nutrient found in eggs, meat, and fish, is another piece of this puzzle. The liver needs choline to package and export fat. Some gut bacteria consume choline before the body can use it, effectively creating a choline deficiency that impairs the liver’s ability to clear fat from its cells.
Medications That Contribute
Certain medications can cause or worsen fatty liver as a side effect. Corticosteroids promote fat accumulation in the liver by mimicking the metabolic effects of excess cortisol. Methotrexate, used for autoimmune conditions, and amiodarone, a heart rhythm medication, are among the drugs most commonly associated with liver fat buildup. Tamoxifen (used in breast cancer treatment), some anti-seizure medications, and certain HIV medications can also contribute. If you’re taking a long-term medication and develop fatty liver, the medication itself may be a factor worth discussing with your prescriber.
How Simple Fat Becomes Serious Disease
Fat sitting quietly in liver cells is considered relatively benign on its own. The danger comes when that stored fat begins to damage cells, a process called lipotoxicity. Excess fat inside liver cells stresses internal structures, generates reactive oxygen molecules, and eventually triggers cell death. The liver responds with inflammation, recruiting immune cells that cause further damage.
This transition from simple fatty liver to the inflammatory form (called MASH, previously NASH) is what drives disease progression toward scarring and, in some cases, cirrhosis or liver cancer. The process isn’t inevitable. Many people live with stable fatty liver for years or decades without progressing. But the more risk factors you have, particularly insulin resistance, obesity, the PNPLA3 gene variant, and ongoing dietary or alcohol triggers, the higher the likelihood of progression. Losing 7 to 10% of body weight has been shown to reduce liver fat significantly and can reverse even the inflammatory stage in many cases.