- The main driver of fatty liver (MASLD) is insulin resistance, usually alongside extra calories and belly fat.
- About 59% of liver fat comes from fat released by body fat stores, and about 26% is made in the liver from sugar and starch.
- Genes such as PNPLA3, alcohol, some medicines and very rapid weight loss add to the risk.
- You do not have to be heavy: about 1 in 5 people with fatty liver has a lean BMI, and this is common in South Asians.
Fatty liver happens when more fat flows into the liver, or is made there, than the liver can burn or send out. A little fat in the liver is normal. When fat fills more than about 5% of liver cells, it becomes fatty liver disease. Today most of it is linked with metabolism, which is why doctors renamed it metabolic dysfunction-associated steatotic liver disease (MASLD), replacing the older term NAFLD. When inflammation and cell injury join the fat, it is called MASH (formerly NASH).
Under the new definition, MASLD means fat in the liver plus at least one metabolic risk factor, such as a large waist, raised blood sugar, high blood pressure, high triglycerides or low HDL. This article looks at each driver in turn, so you can see which ones apply to you and which ones you can change. For the basics, see Fatty Liver (MASLD) Explained.
Where liver fat comes from
A classic study used labeled fats to trace exactly where the fat in fatty livers came from. The answer surprised many people: most of it was not eaten directly.
Donnelly et al., J Clin Invest 2005 (people with fatty liver, fasting state).
This explains why the main drivers are insulin resistance and belly fat, which release fat into the blood, and excess sugar and refined carbohydrate, which the liver turns into new fat.
1. Insulin resistance: the central driver
Insulin normally tells fat cells to hold on to their fat, and tells the liver to stop making glucose. When the body becomes resistant to insulin, fat cells leak fatty acids into the blood, and the pancreas pumps out more insulin to compensate. High insulin in turn switches on fat-making in the liver. The liver gets hit from both directions. A fatty liver then becomes more insulin resistant itself, creating a cycle that also pushes up blood sugar and triglycerides. Learn more in Insulin Resistance Explained.
The fatty liver cycle
2. Extra calories and belly fat
Eating more energy than you use, over months and years, is the most common starting point. Where the extra fat is stored matters. Visceral fat, the deep fat around the organs, drains straight to the liver and is far more linked with fatty liver than fat on the hips or thighs. A growing waist is often a better warning sign than the number on the scale. Read more in Visceral Fat and Metabolic Health.
Know your energy needs. Our Calorie Calculator gives a rough daily target, and the Waist-to-Height Ratio Calculator shows whether your waist is in the higher-risk range.
3. Sugar, especially fructose
Fructose, the sugar in table sugar, honey, fruit juice and many sweetened drinks, is handled almost entirely by the liver. Unlike glucose, it bypasses a key control step, so large amounts are readily turned into fat. Sugary drinks are the biggest concern because they deliver a lot of fructose quickly and do not fill you up. Whole fruit is different: its fiber and water slow absorption, and it is not linked with fatty liver in the same way. Refined starches such as white rice, maida and white bread also raise insulin and feed liver fat-making when eaten in large portions.
We look at this more closely in Sugar, Fructose and Fatty Liver.
4. Genes: why risk runs in families
Genes explain why two people with the same weight and diet can have very different livers. The best known is a common variant in a gene called PNPLA3. In the study that discovered it, people carrying two copies of the variant had more than twice as much liver fat as people without it. The variant is also linked with more inflammation and scarring. Other genes, such as TM6SF2, play a smaller role. Genetic testing is not routine, but a parent or sibling with cirrhosis is a good reason to get checked.
5. Lean fatty liver and South Asians
Fatty liver is not only a disease of obesity. A large meta-analysis found that about 19% of people with fatty liver had a lean BMI and about 41% were not obese. People of South Asian heritage tend to store more fat around the organs and develop insulin resistance at lower body weights, so fatty liver often appears at a “normal” BMI. For this reason, Asian BMI cut-offs are lower (overweight from 23 and obesity from 25), and waist size is especially useful.
Ye et al., Lancet Gastroenterol Hepatol 2020 (global meta-analysis).
6. Alcohol, medicines and other causes
Alcohol is processed by the liver and adds to fat build-up. The new nomenclature created a category for people who have metabolic risk factors and also drink more than lightly: MetALD. This covers roughly 140–350 g of alcohol a week for women and 210–420 g for men (about 20–50 g a day for women and 30–60 g a day for men). Above those amounts, alcohol is treated as the main cause. For liver health, less is better, and some people should not drink at all; see Coffee, Alcohol and Liver Health.
| Other contributor | What to know |
|---|---|
| Some medicines | Steroid tablets, tamoxifen, methotrexate, amiodarone and sodium valproate can cause or worsen liver fat. |
| Very rapid weight loss | Crash diets and long fasts can sometimes worsen liver inflammation; steady loss is safer. |
| Linked conditions | Type 2 diabetes, polycystic ovary syndrome, an underactive thyroid and sleep apnea all raise the risk. |
| Inactivity | Muscles that are rarely used take up less glucose, leaving more for the liver to turn into fat. |
Do not stop a prescribed medicine because it is on a list like this. For many people the benefit clearly outweighs the liver effect. Talk to your doctor, who may monitor your liver or consider an alternative.
MythFatty liver comes from eating fatty food.
FactOnly about 15% of liver fat comes directly from recent meals. Insulin resistance, belly fat and excess sugar and refined carbs matter far more.
Common questions
Can you get fatty liver without drinking alcohol?
Yes. MASLD is, by definition, driven by metabolic factors, and many people who never drink have it.
Is fruit bad for a fatty liver?
Whole fruit in normal portions is fine and often helpful because of its fiber. The concern is added sugar and fruit juice, which deliver concentrated fructose.
I am slim. Why do I have fatty liver?
Genes, belly fat hidden inside a normal weight, inactivity, a diet high in refined carbs and sugar, or South Asian heritage can all cause it. Lean fatty liver still deserves attention, including a check for scarring.
- Insulin resistance sits at the center of fatty liver, fueled by extra calories and belly fat.
- Most liver fat comes from body fat stores and from sugar and starch turned into fat, not directly from fatty food.
- Sugary drinks, refined carbs, inactivity, genes (PNPLA3), alcohol and some medicines add to risk.
- Lean people, especially South Asians, can have fatty liver; watch your waist, not just your weight.
- Next: Fatty Liver Tests: Liver Enzymes, FIB-4 and Scans Explained.
Sources
- Rinella ME, Lazarus JV, Ratziu V, et al. A multisociety Delphi consensus statement on new fatty liver disease nomenclature. Hepatology. 2023;78(6):1966–1986.
- Donnelly KL, Smith CI, Schwarzenberg SJ, et al. Sources of fatty acids stored in liver and secreted via lipoproteins in patients with nonalcoholic fatty liver disease. J Clin Invest. 2005;115(5):1343–1351.
- Romeo S, Kozlitina J, Xing C, et al. Genetic variation in PNPLA3 confers susceptibility to nonalcoholic fatty liver disease. Nat Genet. 2008;40(12):1461–1465.
- Jensen T, Abdelmalek MF, Sullivan S, et al. Fructose and sugar: a major mediator of non-alcoholic fatty liver disease. J Hepatol. 2018;68(5):1063–1075.
- Ye Q, Zou B, Yeo YH, et al. Global prevalence, incidence, and outcomes of non-obese or lean non-alcoholic fatty liver disease: a systematic review and meta-analysis. Lancet Gastroenterol Hepatol. 2020;5(8):739–752.
- Rinella ME, Neuschwander-Tetri BA, Siddiqui MS, et al. AASLD Practice Guidance on the clinical assessment and management of nonalcoholic fatty liver disease. Hepatology. 2023;77(5):1797–1835.
- European Association for the Study of the Liver (EASL), European Association for the Study of Diabetes (EASD), European Association for the Study of Obesity (EASO). EASL-EASD-EASO Clinical Practice Guidelines on the management of metabolic dysfunction-associated steatotic liver disease (MASLD). J Hepatol. 2024;81(3):492–542.
This article is for general education and is not a substitute for advice from your own doctor or healthcare team.