Yes, being underweight cause fatty liver: The surprising truth about lean NAFLD

December 24, 2025 •

4 min read

Studies show that up to 20% of people with fatty liver disease are not overweight, challenging the long-held assumption that only excess weight causes this condition. The truth is, being underweight can cause fatty liver, a diagnosis now referred to as Metabolic Dysfunction-Associated Steatotic Liver Disease (MASLD) when not linked to alcohol.

Quick Summary

Lean non-alcoholic fatty liver disease (NAFLD) affects individuals with a healthy or underweight BMI due to factors like excess visceral fat, poor nutrition, insulin resistance, and genetic predispositions. This article explores the mechanisms and risks behind fat accumulation in the liver, independent of overall body weight.

Key Points

Underweight individuals can develop fatty liver: This condition, known as lean NAFLD, is a metabolic issue that challenges the assumption that only obese people are at risk.
Visceral fat is a major risk factor: People who are 'thin on the outside, fat on the inside' (TOFI) can accumulate dangerous visceral fat around their organs, leading to liver damage.
Poor diet quality contributes significantly: Diets high in refined carbohydrates, sugar, and unhealthy fats can cause liver fat buildup even in lean individuals. Malnutrition and choline deficiency are also implicated.
Genetics can predispose individuals: Specific gene variants, like PNPLA3 and TM6SF2, increase the risk of fatty liver regardless of a person's body weight.
Sarcopenia can worsen the condition: Loss of muscle mass (sarcopenia) is linked to lean NAFLD and impairs metabolic health, creating a cycle that promotes liver fat accumulation.
Lifestyle changes are the primary treatment: A balanced diet focusing on whole foods, regular exercise (including strength training), and avoiding alcohol are key for managing lean NAFLD.

Understanding the 'Lean Paradox'

Fatty liver, or hepatic steatosis, is characterized by a buildup of fat in the liver cells. While a well-known risk factor is obesity, a surprising number of individuals with a healthy or low Body Mass Index (BMI) are being diagnosed with what was previously known as lean NAFLD. This phenomenon is often termed the 'lean paradox' and highlights that overall body weight is not the sole determinant of liver health. In some Asian populations, for example, studies have indicated that as many as 40% of fatty liver cases occur in people with a normal BMI. The condition is a metabolic issue, not simply a weight issue.

Key Factors Contributing to Fatty Liver in Underweight Individuals

For individuals with a low BMI, the development of fatty liver is driven by several mechanisms that differ from obesity-related causes. These factors can operate independently of, or in combination with, overall body weight.

Excessive Visceral Fat

Some people may be 'thin on the outside, fat on the inside' (TOFI). This means they carry excessive visceral fat, the dangerous fat stored around internal organs like the liver, despite appearing thin. This metabolically active fat releases inflammatory chemicals and hormones that disrupt normal function and promote fat accumulation in the liver, leading to MASLD.

Poor Diet and Malnutrition

Undernutrition, malnutrition, and rapid weight loss can paradoxically lead to fatty liver. Diets high in refined carbohydrates, sugar-sweetened beverages, and unhealthy fats can overload the liver, leading to fat storage, even when overall calorie intake is low. A deficiency in essential nutrients, such as choline (often found in animal products), can also impair lipid metabolism and contribute to liver damage.

Insulin Resistance

Even without obesity, some lean individuals can develop insulin resistance. This condition, where the body's cells don't respond effectively to insulin, disrupts glucose and fat metabolism, forcing more fat to be stored in the liver. This creates a state of metabolic dysfunction that mirrors the pathways seen in obese individuals with fatty liver.

Genetic Predisposition

Genetics play a significant role in determining an individual's susceptibility to fatty liver. Certain genetic variants, such as those in the PNPLA3 and TM6SF2 genes, have been strongly linked to liver fat accumulation and disease progression, independent of body size. This can explain why some people remain healthy despite an unhealthy lifestyle, while others develop liver disease despite maintaining a healthy weight.

Sarcopenia (Muscle Loss)

Sarcopenia, the progressive loss of muscle mass and function, is commonly associated with lean NAFLD. Skeletal muscle plays a crucial role in glucose and energy metabolism. The loss of muscle tissue can impair insulin sensitivity and worsen metabolic health, creating a vicious cycle that encourages ectopic fat accumulation in the liver.

Comparison: Lean vs. Obese Fatty Liver

While the outcome can be similar, the underlying pathways differ. Here is a comparative overview:


Feature	Lean Fatty Liver (Lean NAFLD)	Obese Fatty Liver (Classical NAFLD)
Body Mass Index (BMI)	Normal or Underweight (<25 kg/m² or <23 kg/m² for Asians)	Overweight or Obese (>25 kg/m²)
Primary Fat Type	Often higher proportion of visceral fat	Higher total body fat, including subcutaneous and visceral
Main Metabolic Driver	Greater influence from genetics, visceral fat, and insulin sensitivity issues	Strong link to excess calorie intake and body weight
Key Dietary Factors	High fructose/sugar, unhealthy fats, and malnutrition	High calorie intake and unhealthy dietary patterns
Muscle Mass	Often associated with low muscle mass (sarcopenia)	Muscle mass varies, but sarcopenic obesity is possible
Risk Perception	Often overlooked and under-diagnosed due to 'healthy' appearance	More frequently screened and diagnosed due to visible risk factor

Prevention and Management Strategies

Regardless of your weight, managing fatty liver involves a combination of lifestyle changes focused on diet and exercise. If you are underweight, the goal is not weight loss, but rather to improve metabolic function and reduce liver fat without causing further weight loss.

Here are some effective strategies:

Improve Diet Quality: Emphasize a balanced intake of complex carbohydrates, lean protein, and healthy fats. Focus on whole grains, fruits, vegetables, and lean sources of protein while reducing consumption of refined carbohydrates, sugary drinks, and processed foods. Addressing any nutritional deficiencies, such as choline, is also important.
Regular Physical Activity: Aim for regular exercise to improve insulin sensitivity and reduce fat in the liver. A combination of aerobic exercise and strength training can help build muscle mass, addressing sarcopenia, which is linked to lean NAFLD.
Limit Alcohol: Even small amounts of alcohol can worsen liver damage in individuals with fatty liver disease. Limiting or avoiding alcohol is a critical step for recovery.
Consult a Professional: A doctor or registered dietitian can help create a customized plan that addresses the specific nutritional needs of an underweight person with fatty liver, ensuring healthy fat loss from the liver without causing unhealthy overall weight loss.

Conclusion

While the association between obesity and fatty liver is widely recognized, the role of being underweight is often misunderstood. The concept of lean NAFLD demonstrates that fat accumulation in the liver is a metabolic, not just a weight-related, issue. Factors such as genetics, visceral fat distribution, poor diet quality, and insulin resistance can contribute to liver fat even in those with a low BMI. By understanding these underlying causes, individuals can take proactive steps to improve their liver health through targeted dietary improvements, regular exercise, and professional medical guidance. The key is to focus on metabolic health rather than simply the number on the scale.

For more in-depth information, the National Institutes of Health (NIH) provides extensive research and resources on non-alcoholic fatty liver disease. NIH Nonalcoholic Fatty Liver Disease

Frequently Asked Questions

While less recognized than in obese populations, lean non-alcoholic fatty liver disease (NAFLD) is not uncommon. Studies show that up to 20% of people with NAFLD are not overweight, and in some regions, a significant portion of cases are in those with a normal or low BMI.

The main difference lies in the underlying risk factors. Obese fatty liver is heavily influenced by excess body weight and calorie intake, whereas lean fatty liver is more driven by visceral fat distribution, genetics, poor diet quality, and insulin resistance, independent of overall weight.

Yes, malnutrition can lead to fatty liver. Inadequate intake of protein and other essential nutrients can impair the liver's ability to metabolize fats correctly, causing them to accumulate. Conditions like rapid weight loss also trigger this process.

TOFI describes individuals who appear slim but have a high amount of visceral fat stored around their internal organs. This hidden fat is metabolically dangerous and increases the risk of conditions like fatty liver and diabetes, even with a normal BMI.

Not necessarily. While some studies show a milder initial metabolic profile in lean NAFLD patients, the risk of disease progression to serious conditions like fibrosis, cirrhosis, and higher mortality rates is comparable to or sometimes greater than in obese patients.

The goal is to improve metabolic health, not lose more weight. A recommended diet includes balanced portions of whole grains, lean protein, healthy fats, and plenty of fruits and vegetables while limiting refined carbohydrates, sugary foods, and processed items. A healthcare professional can help tailor a nutritional plan.

Yes, genetic factors are known to increase susceptibility. Specific gene variants, such as PNPLA3 and TM6SF2, have been identified that can predispose an individual to liver fat accumulation regardless of their body size.