Fatty liver disease develops when fat accumulates in liver cells faster than the liver can process or export it. Around 1.3 billion people worldwide, roughly 16% of the global population, are living with the condition as of 2023. The causes range from what you eat and drink to your genetics, medications, and underlying metabolic health.
The Two Main Types
Fatty liver disease falls into two broad categories based on what’s driving the fat buildup. The most common form, now officially called metabolic dysfunction-associated steatotic liver disease (MASLD), is linked to metabolic problems like insulin resistance, obesity, and high blood sugar. You may still see it referred to by its older name, nonalcoholic fatty liver disease (NAFLD). The other major type is alcohol-related liver disease, caused by heavy drinking over time. Both types start with excess fat in the liver and can progress to inflammation, scarring, and eventually serious liver damage.
A diagnosis of MASLD requires fat in the liver plus at least one cardiometabolic risk factor: a BMI of 25 or higher, elevated fasting blood sugar or type 2 diabetes, high blood pressure, high triglycerides, or low HDL cholesterol. Most people with fatty liver have several of these at once.
Insulin Resistance: The Central Driver
Insulin resistance is the single most important metabolic cause of fatty liver. When your cells stop responding normally to insulin, your body compensates by producing more of it. That excess insulin signals fat tissue to release more fatty acids into the bloodstream, and those fatty acids flood the liver. At the same time, high insulin levels tell the liver to ramp up its own fat production.
Inside liver cells, the buildup of a specific fat molecule called diacylglycerol (DAG) appears to be the key link between liver fat and worsening insulin resistance. Research published in the Proceedings of the National Academy of Sciences found that DAG content in liver cells was the single best predictor of insulin resistance, accounting for 64% of the variability in insulin sensitivity across study participants. This creates a vicious cycle: insulin resistance drives fat into the liver, and the accumulating fat makes insulin resistance worse.
You don’t need to be visibly overweight to develop insulin resistance. People at a normal weight can still carry excess fat in their liver if they have other metabolic risk factors, a pattern sometimes called “lean fatty liver.”
How Diet Contributes
Eating more calories than your body uses is the broadest dietary risk factor, but the type of calories matters too. Fructose, the sugar found naturally in fruit but consumed in much larger quantities through sweetened drinks, processed foods, and table sugar, is uniquely harmful to the liver.
Unlike glucose, which every cell in your body can use for energy, fructose is processed almost entirely by the liver. That processing happens faster than glucose metabolism, which can deplete the liver’s energy reserves. More importantly, fructose powerfully activates the liver’s fat-making machinery. It switches on genes responsible for building new fatty acids and does so more aggressively than glucose. Animal studies show that fructose feeding increases the activity of the very first enzyme in fructose processing, and the same enzyme is elevated in people with inflammatory fatty liver disease.
Diets high in refined carbohydrates and saturated fat also contribute, partly because excess carbohydrates are converted to fat in the liver through a process called de novo lipogenesis. Your liver takes building blocks from carbohydrate metabolism, assembles them into fatty acids, and packages those fatty acids into fat droplets for storage. When this process outpaces the liver’s ability to export or burn that fat, steatosis develops.
How Alcohol Causes Fatty Liver
Alcohol-related fatty liver operates through a different but overlapping set of mechanisms. When your liver breaks down alcohol, the chemical reactions involved shift the liver’s internal chemistry in a way that directly blocks fat burning. Specifically, alcohol metabolism consumes a molecule the liver needs for burning fatty acids, forcing the liver to prioritize processing alcohol over processing fat. The result is that dietary fat and fat already stored in the liver simply accumulates.
Chronic drinking also disrupts a key cellular energy sensor that normally keeps fat production and fat burning in balance. When this sensor is impaired, the liver simultaneously makes more fat and burns less of it. This is why even moderate but consistent alcohol intake over years can lead to measurable fat buildup in the liver, and why heavy drinking accelerates the process dramatically. The inhibition of fat burning in the mitochondria, the cell’s energy-producing structures, is considered the most significant contributor to alcohol-related liver fat accumulation.
Genetic Risk Factors
Your genes play a meaningful role in how easily fat builds up in your liver. The most well-studied genetic variant is a mutation in the PNPLA3 gene, known as I148M. This variant is carried by roughly one in three people of Hispanic descent, making it one reason fatty liver rates are higher in that population.
Normally, the PNPLA3 protein helps break down fat stored in liver cell droplets. The I148M variant produces a version of the protein that doesn’t work properly. Instead of helping clear fat, it interferes with the enzymes that break triglycerides down. In laboratory studies, cells carrying this variant accumulated fat droplets with roughly 1.4 times the mass and area of normal cells under baseline conditions, and the difference grew to at least 1.6 times when the cells were exposed to fatty acids. The variant doesn’t guarantee you’ll develop fatty liver, but it lowers the threshold, meaning you need fewer additional risk factors before fat starts accumulating.
Other genetic variants affecting fat metabolism, sugar processing, and liver cell membrane composition have also been linked to increased risk, though their individual effects are smaller than PNPLA3.
Medications That Can Cause Fatty Liver
Certain medications can trigger fat buildup in the liver as a side effect. The best-known culprits include amiodarone (a heart rhythm drug), corticosteroids, tamoxifen (used in breast cancer treatment), and some chemotherapy agents like cisplatin. These drugs interfere with the liver’s mitochondria, disrupting energy production and fat processing in ways that mimic the metabolic dysfunction seen in other forms of fatty liver.
Steroid hormones can worsen the same pathways that drive metabolic fatty liver, while other drugs appear to unmask fatty liver that was already developing silently. If you’re taking a long-term medication and have other risk factors, your doctor may monitor your liver health with periodic blood tests or imaging.
Rapid Weight Loss and Malnutrition
Paradoxically, losing weight too quickly can cause fatty liver rather than cure it. When the body is starved of calories, whether through crash dieting, bariatric surgery complications, or actual malnutrition, it rapidly breaks down stored body fat and sends a flood of fatty acids to the liver. The liver can’t process this sudden surge fast enough, and fat accumulates. Severe malnutrition and total parenteral nutrition (IV feeding that bypasses the gut) can trigger the same effect.
This is one reason doctors typically recommend gradual weight loss of one to two pounds per week for people with fatty liver. Slow, steady weight loss gives the liver time to process the mobilized fat and actually reduces liver fat content over time, while extreme calorie restriction can temporarily make things worse.
What Happens as Fat Accumulates
Simple fat accumulation in the liver is the earliest stage and is often reversible. The trouble starts when the liver’s energy-producing structures, the mitochondria, begin to fail under the strain. In early fatty liver, mitochondria actually work harder to try to burn off excess fat. But as the disease progresses, they become damaged. Abnormal, oversized mitochondria appear in liver cells, their ability to produce energy drops, and the cell’s cleanup systems that normally remove damaged mitochondria stop working efficiently.
This mitochondrial failure marks the transition from simple fatty liver to the inflammatory stage, where liver cells begin dying and scar tissue starts forming. Not everyone progresses to this point. An estimated 20 to 30% of people with simple steatosis eventually develop the inflammatory form, and the speed of progression depends on the same factors that caused the fat buildup in the first place: ongoing insulin resistance, continued heavy drinking, genetic susceptibility, and diet.
The regional differences in fatty liver prevalence reflect how these causes overlap in different populations. North Africa and the Middle East have the highest rates globally, nearly double the world average, driven by rising obesity, high sugar consumption, and genetic factors. High-income Asia Pacific countries have the lowest rates, at roughly half the global average.