QUESTION: My doctor just told me that I have fatty liver disease. She says that in my case it has nothing to do with drinking alcohol, which makes sense since I rarely drink. But I still don't understand it. She says it has to do with the fact that I've been overweight most of my life.

ANSWER: Nonalcoholic fatty liver disease (NAFLD) was rarely diagnosed before 1980. But today, it is believed to affect as many as 30% of adult Americans.

Nonalcoholic fatty liver disease is actually not a single disease. Instead, it's a spectrum of disorders, all marked by the accumulation of fat inside liver cells. Under the microscope, this fatty buildup looks just like alcohol-induced fatty liver disease, but it occurs in people who consume little or no alcohol.

Nonalcoholic fatty liver disease usually causes no symptoms and few, if any, complications. However, some people go on to develop serious problems.

The exact cause of NAFLD is unknown. But its emergence as a recognized and increasingly common disorder coincides with the epidemics of obesity and Type 2 diabetes. Both of these are associated with insulin resistance.

Obesity and insulin resistance, in turn, are implicated in metabolic syndrome, a group of risk factors that increase the risk of cardiovascular disease, including a large waist and high triglycerides (a type of blood fat). Some experts consider nonalcoholic fatty liver disease a symptom of metabolic syndrome.

The leading theory about nonalcoholic fatty liver disease is that it's a result of metabolic changes driven by insulin resistance. More fat stays in the liver, accumulating in tiny sacs in the liver cells.

In its simplest form, nonalcoholic fatty liver disease is just fat in the liver cells. This condition is called steatosis. While steatosis is not normal, it causes no symptoms and by itself is usually harmless. However, 5% to 20% of people with steatosis will develop a more serious form of nonalcoholic fatty liver disease called nonalcoholic steatohepatitis, or NASH. In NASH, the fattened cells cause inflammation, swelling and cell death.

We don't know the cause of this more serious, inflammatory form of nonalcoholic fatty liver disease or who is most likely to develop it, but it is not simply related to the amount of fat in the liver cells.

NASH is usually a relatively stable condition with few symptoms. Most people don't know they have it unless it shows up during a blood test or imaging procedure performed for other reasons. But unlike steatosis, NASH is not harmless.

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