By Peter G. Traber, M.D. on April 21, 2016
I suppose it’s natural to assume that only overweight people get fatty liver disease (and NASH), but one thing I have learned over my career in medicine is to always challenge such “common sense” assumptions with empirical research. It turns out that, while people who are overweight or obese are indeed at greater risk, thin people can also develop fatty liver disease, NASH and cirrhosis. This has come to be known as “lean NASH.”
The commonly accepted definition of obesity is a body mass index (BMI) of 30 or more. However, overall BMI is less important in determining risk for fatty liver disease than where the fat is located in the body. Visceral fat, when the fat is nestled in and around the organs of the belly, is more strongly linked to fatty liver disease and other metabolic disorders than fat in arms, legs, and other parts of the body. A person can easily have a BMI well below 30 and still have considerable visceral fat. Someone who is “metabolically obese, normal weight” will have many of the hallmarks of obesity, such as insulin resistance (requirement of higher insulin levels to control blood sugar), metabolic syndrome (high blood sugar, elevated fats and cholesterol in blood, high blood pressure, and excess body fat around the waist) and NASH.
A study published in 2012 showed that overall prevalence of fatty liver disease among obese individuals was 28 percent, while fatty liver disease was identifiable in 7 percent of the lean individuals tested. (Zobair M. Younossi, 2012) Yes, obese people had a greater incidence of fatty liver disease, but lean people still showed a surprisingly high prevalence of the disease.
The other surprising finding from this study was that lean NASH patients here in the U.S. tend to be Hispanic. It’s unclear whether it is culture, diet, genetics, or some completely different mechanism at work that makes those of Hispanic ancestry more likely to develop lean NASH. But, given the growth of the Hispanic population here in the U.S., lean NASH is likely to emerge in the coming decades as an important cause of chronic liver disease.
Ethnicity does seem to be one of the major determinants of lean NASH, which was first described by physicians in Asia. While metabolic syndrome has long been a problem in developed countries, it is an increasing problem in developing countries as well, even though the rate of obesity remains comparatively low. The prevalence of fatty liver disease among normal-weight individuals was recently reported at 12% in Greece, 20% in India and 15% in China.
One analysis from 2013 suggests that lean NASH, as seen in Asia, is a distinct phenotype of NASH (Kausik Das, 2013). Asians, this study notes, show a propensity to develop metabolic syndrome at a lower BMI. One possible reason is that early malnutrition, either in utero or in early childhood, primes the body to store visceral fat more aggressively. The relative abundance of food in Asia today over the scarcities common only a few decades ago means that adults in China, India and other Asian countries are increasingly at risk of developing lean NASH.
I’ve called fatty liver disease a “hidden epidemic,” and lean NASH is even more so. It is easy to identify someone with a high body weight as being at risk for fatty liver disease, but less so for someone with lean NASH. We need a much better understanding of what causes lean NASH and how its presentation and biomarkers are distinct from the fatty liver disease and NASH seen in overweight patients.
Works Cited
- Kausik Das, A. C. (2013). Lean NASH: distinctiveness and clinical implication. Hepatol Int , 7(Supplement 2), S806 – S813.
- Zobair M. Younossi, M. M. (2012, November). Nonalcoholic Fatty Liver Disease in Lean Individuals in the United States. Medicine, 91(6), 319-227.
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