Researchers from Johns Hopkins University School of Medicine tackled this question in a 2011 study, published in the British Medical Journal.
Lead scientist Mariana Lazo, an assistant professor of Medicine and Epidemiology at Johns Hopkins, says the team expected to discover how extensively non-alcoholic fatty liver disease affects the death rates of those who have it.
The disease was first named in 1980, after doctors had found a growing number of obese or diabetic patients had excess fat in their livers. The condition was mysterious, because although patients insisted they were not drinking to excess, their livers had damage virtually identical to that found in alcoholics. Since then, researchers have discovered that while some fat in the liver is healthy and seems to pose no problem, once the amount of fat reaches 5-10% of the organ's total weight (the benchmark for fatty liver disease), it is much more vulnerable to damage like swelling, scarring, and liver failure.
Though the exact cause of the disease is not known, there's evidence that eating too much food, or the wrong food (that is, highly-processed, high-fat food without proper nutrition), plays a major role.
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Using data collected between 1994 and 1998 on 11,371 American men and women, the Johns Hopkins scientists determined which subjects had fatty livers. To their amazement, there appeared to be no excess risk of death after up to 18 years, among the adults with fatty liver disease.
Mariana Lazo is still at a loss to explain her team's findings, calling the results "shocking".
"We know that the liver is damaged by fatty liver disease as it progresses," she tells me. "It may be that the body deposits fat in the liver as a kind of protective self-defense mechanism, to save the heart and other organs. But at a certain point, the liver fails, too. We may just not have discovered the point at which that happens."
She speculates that more information will be uncovered in future research. Her team used data on liver enzymes and fat content, initially gathered to assess gallbladder health in the subjects. No biopsies were available to analyze. In advanced liver disease, she says, fat disappears from the organ, so deaths ultimately due to fatty livers may be under-recorded.
In a similar finding, researchers from Queen's University in Kingston, Canada, examined the relationship between belly fat (either visceral or abdominal), and liver fat, with all-cause mortality. Their paper, published in 2006 in the journal Obesity, concluded that of the three risk factors, only visceral fat was a significant predictor of mortality (that is, associated with a greater risk of death).
Mariana Lazo cautions that even without evidence that excess mortality is associated with fatty liver disease, having too much fat in the liver puts it at risk of damage, and should be avoided.
"We have vaccines to prevent viral liver disease like hepatitis," she says. "There's no vaccine against fatty liver disease, and no drug to treat the damage caused by it. Prevention is key."
Jennifer L. Kuk, Peter T. Katzmarzyk, Milton Z. Nichaman, Timothy S. Church, Steven N. Blair, and Robert Ross. "Visceral Fat Is an Independent Predictor of All-cause Mortality in Men." Obesity (2006) 14, 336–341; doi: 10.1038/oby.2006.43.
Lazo M, Hernaez R, Bonekamp S, Kamel IR, Brancati FL, Guallar E, Clark JM. “Non-alcoholic fatty liver disease and mortality among US adults: prospective cohort study.” BMJ. 2011 Nov 18;343:d6891.
Non-alcoholic Fatty Liver Disease (NAFLD). American Liver Foundation Public Information Sheet. Accessed December 19, 2012.