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NASH Definition & Prevalence

What is fatty liver disease?

Fatty liver disease is a condition in which excess fat is stored inside liver cells, making it harder for the liver to function. One cause of fat buildup in the liver is heavy alcohol use, referred to as alcoholic fatty liver disease. This is a common, but preventable disease and is the earliest stage of alcohol-related liver disease. You can read more about the different stages of alcohol-related liver disease here.

When the buildup of fat in the liver is not related to significant alcohol consumption, the condition is called nonalcoholic fatty liver disease (NAFLD).

Healthy Liver v. Fatty Liver

What is nonalcoholic fatty liver disease (NAFLD)?

NAFLD is a general term for a range of conditions characterized by extra fat in liver cells that is not caused by alcohol. It’s normal for the liver to contain some fat. However, if more than 5 percent of the liver’s weight is fat, it’s considered a fatty liver (steatosis). There are two different types of nonalcoholic fatty liver disease:

  • Simple fatty liver
    In this form of NAFLD you have fat in your liver, but little or no inflammation of the liver or damage to liver cells. Your healthcare provider may refer to this as nonalcoholic fatty liver (NAFL). Typically, this form does not progress to cause liver damage.
  • Nonalcoholic steatohepatitis (NASH)
    This is the more severe form of NAFLD in which you have hepatitis – meaning swelling or inflammation of the liver –– and liver cell damage, in addition to fat in your liver. Inflammation and liver cell damage can cause fibrosis, or scarring, of the liver.

    • Fibrosis
      This fibrosis can progress to cirrhosis, where hard scar tissue replaces an increasingly larger amount of soft healthy liver tissue. Cirrhosis from NASH typically takes years of damage to develop.
NASH infographic

How common is NAFLD and NASH?

NAFLD is the most common chronic liver condition in the United States. It’s estimated that about 25 percent of adults in the U.S. have NAFLD. Of those with NAFLD, about 20 percent have NASH (5% of adults in the U.S.). Most people with NAFLD have simply fatty liver.

The reason some people with NAFLD have simple fatty liver and others get NASH isn’t known, although research suggests that certain genes may play a role.

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