What Is Fatty Liver and Why It’s Important for Teens to Understand
What The Liver Does — And Why It’s Not Supposed to Store Fat
The liver is one of the most important organs in your body. It works every minute of every day to filter toxins from your blood, make important proteins, break down nutrients from food, and help manage your body’s energy, glucose (sugar), and cholesterol levels.
What your liver is not meant to do, however, is store fat.
When the liver becomes a storage space for fat (known as “fatty liver”), it has a hard time doing all parts of its job. Fatty liver in youth is more than a liver problem — it's a warning sign of deeper metabolic issues.
Thanks to the participation of numerous kids and teens in many research studies at Yale, scientists identified the risk factors and the dangers of fatty liver, especially for youth living with bigger bodies.
What Raises the Risk of Fatty Liver in Youth?
Over many years, researchers identified factors that make it more likely for someone to get fatty liver, including:
Higher fat mass
Your body needs to store fat molecules, so the molecules don’t drift around in the blood and cause problems. The more fat molecules your body has, the more likely your body will be to use the liver as a place for fat storage. Even though it’s not ideal, your body may not have a choice.
High insulin and insulin resistance
One of the strongest risk factors for fatty liver is insulin resistance, when the body doesn’t respond well to insulin. Two studies found that youth with fatty liver had very high insulin resistance, even more so than those with similar body weights but no liver fat. The youth with fatty liver were also making more insulin.
A vicious cycle can start. Liver fat may cause or worsen insulin resistance by disrupting how insulin works in the liver.
Lots of ups and downs of glucose
Even small swings in blood sugar (also known as “glucose variability”), when they happen every day, can lead to higher liver fat and more stress on the liver.
Genetics, ancestry, and ethnicity
More than 500 teens with bigger bodies had imaging of their liver. Genetic background and ethnicity made a difference. Fatty liver was present in 60% Hispanic teens and 42% Caucasian teens, while only 16% of African American teens had it. In other words, Black teens are much less likely to develop fatty liver than white or Hispanic teens, even when they have similar body types.
Yet, the black teens who developed fatty liver had worse symptoms than white or Hispanic teens. Their bodies had a harder time processing sugars from food and controlling their insulin levels. Meaning, Black teens are less likely to have fatty liver, but when they do, the effects are worse.
Why Is Liver Fat Dangerous?
Fat in the liver may seem invisible because we don’t see or feel it. Though it starts quietly, fatty liver can have serious consequences. A 2020 study of more than 280 kids and teens discovered that liver fat is significantly more dangerous than fat stored in other parts of the body, including under the skin or in the belly.
Here are some of these risks.
Diabetes Risk: A Chain Reaction
Fatty liver plays a big role in developing prediabetes and type 2 diabetes.
Many studies have found that fat in the liver, and all the problems associated with it, shows up before teens are diagnosed with type 2 diabetes. Youth with liver fat often have glucose spikes that last a long time after eating—even if their fasting glucose seems normal.
How?
- Liver fat makes the liver less responsive to insulin. The liver can’t listen to insulin’s message to absorb glucose, store glucose, and stop making glucose.
- Liver fat causes insulin resistance. Muscles and fat tissue have a harder time soaking up glucose from the blood because they can’t listen to insulin’s message to open their cells to glucose. This leads to a build-up of glucose in the blood, which is dangerous.
- Liver fat also makes it harder for the pancreas to make enough insulin fast enough when we first start eating. This means glucose from our food will be higher in the bloodstream for a longer time.
Liver Releases an Unhealthy Mix of Fat into Blood
Research found that fat in the liver causes an unhealthy mix of fat particles in the blood because the liver can’t properly organize and release these particles.
This mix is known as ‘dyslipidemia’, and includes:
- Less HDL (“good” cholesterol)
- More LDL (“bad” cholesterol)
- More triglycerides (a type of fat molecule)
This unhealthy mix of fat particles can block important blood vessels and lead to heart attacks and strokes, even in teens and young adults.
Inflammation Throughout the Body
More than 100 teens with bigger bodies donated blood to better understand the impact of liver fat. Researchers noticed that having high liver fat triggered the release of “proinflammatory” chemicals that circulate throughout the body.
These chemicals lead to general inflammation everywhere and are linked to:
- Fatigue or tiredness
- Worsening insulin resistance
- Progression to a more dangerous liver condition, called metabolic-associated steatohepatitis (MASH)
Long-Term Problems: Liver Damage, Failure, and Cancer
If left unchecked, fat in the liver slowly destroys the liver.
This destruction progresses over time, like this:
- Inflammation and scarring (known as fibrosis) in the liver
- Permanent damage to the liver structure and ability to work (known as cirrhosis)
- Liver cancer (known as hepatocellular carcinoma)
What starts as silent buildup can end in serious, even life-threatening, disease.
It Matters Where Fat is Stored
Our health is not solely determined by the amount of fat that we hold in our body. It’s more about where that fat is stored – whether that’s under our skin, surrounding our organs, or in our liver. If it’s stored in the liver, it becomes particularly worrisome.
The good news? Fatty liver can often be improved — or even reversed — with the right care.
Instead of aiming for a specific weight, you can support your liver with a balanced approach to food, physical activity that works for your body, and regular support from a doctor you trust.
This report was written by Abigail Kelley and Clare Flannery, MD
Scientific Studies
Intrahepatic fat accumulation and alterations in lipoprotein composition in obese adolescents: a perfect proatherogenic state. Cali AM, Zern TL, Taksali SE, de Oliveira AM, Dufour S, Otvos JD, Caprio S. Diabetes Care. 2007 Dec;30(12):3093-8. doi: 10.2337/dc07-1088. Epub 2007 Aug 23. PMID: 17717283
Central role of fatty liver in the pathogenesis of insulin resistance in obese adolescents. D'Adamo E, Cali AM, Weiss R, Santoro N, Pierpont B, Northrup V, Caprio S. Diabetes Care. 2010 Aug;33(8):1817-22. doi: 10.2337/dc10-0284. PMID: 20668154 Free PMC article.
Glucose dysregulation and hepatic steatosis in obese adolescents: is there a link? Cali AM, De Oliveira AM, Kim H, Chen S, Reyes-Mugica M, Escalera S, Dziura J, Taksali SE, Kursawe R, Shaw M, Savoye M, Pierpont B, Constable RT, Caprio S. Hepatology. 2009 Jun;49(6):1896-903. doi: 10.1002/hep.22858. PMID: 19434725 Free PMC article.
The association between hepatic fat content and liver injury in obese children and adolescents: effects of ethnicity, insulin resistance, and common gene variants. Santoro N, Feldstein AE, Enoksson E, Pierpont B, Kursawe R, Kim G, Caprio S. Diabetes Care. 2013 May;36(5):1353-60. doi: 10.2337/dc12-1791. Epub 2012 Dec 28. PMID: 23275357 Free PMC article.
Longitudinal effects of MRI-measured hepatic steatosis on biomarkers of glucose homeostasis and hepatic apoptosis in obese youth. Kim G, Giannini C, Pierpont B, Feldstein AE, Santoro N, Kursawe R, Shaw M, Duran E, Goldberg R, Dziura J, Caprio S. Diabetes Care. 2013 Jan;36(1):130-6. doi: 10.2337/dc12-0277. Epub 2012 Aug 28. PMID: 22933439 Free PMC article.
Metabolic Features of Nonalcoholic Fatty Liver (NAFL) in Obese Adolescents: Findings From a Multiethnic Cohort. Tricò D, Caprio S, Rosaria Umano G, Pierpont B, Nouws J, Galderisi A, Kim G, Mata MM, Santoro N. Hepatology. 2018 Oct;68(4):1376-1390. doi: 10.1002/hep.30035. PMID: 29665034 Free PMC article.
Hepatic fat is a stronger correlate of key clinical and molecular abnormalities than visceral and abdominal subcutaneous fat in youth. Cioffi CE, Narayan KMV, Liu K, Uppal K, Jones DP, Tran V, Yu T, Alvarez JA, Bellissimo MP, Maner-Smith KM, Pierpoint B, Caprio S, Santoro N, Vos MB. BMJ Open Diabetes Res Care. 2020 Jul;8(1):e001126. doi: 10.1136/bmjdrc-2019-001126. PMID: 32699106 Free PMC article.
Intrahepatic fat, irrespective of ethnicity, is associated with reduced endogenous insulin clearance and hepatic insulin resistance in obese youths: A cross-sectional and longitudinal study from the Yale Pediatric NAFLD cohort. Tricò D, Galderisi A, Mari A, Polidori D, Galuppo B, Pierpont B, Samuels S, Santoro N, Caprio S. Diabetes Obes Metab. 2020 Sep;22(9):1628-1638. doi: 10.1111/dom.14076. Epub 2020 May 31. PMID: 32363679 Free PMC article.
Daily glucose variability is associated with intrahepatic fat content, β cell sensitivity, and biomarkers of glycolysis in youth with obesity. Barbieri E, Bonet J, Fox D, Nelson R, Nelson MB, Nelson L, Fernandez C, Van Name M, Samuels S, Caprio S, Sabati M, Galderisi A, Sherr J, Dalla Man C, Santoro N. Obesity (Silver Spring). 2025 Jan;33(1):116-124. doi: 10.1002/oby.24175. Epub 2024 Dec 10. PMID: 39658509