Nonalcoholic steatohepatitis (NASH) is a part of a group of liver diseases known as nonalcoholic fatty liver disease (NAFLD). The buildup of fat in the cells of the liver is called fatty liver, or steatosis, and in itself is not harmful. Many people have fatty liver. In NASH, the fat buildup causes inflammation of the liver, which can lead to symptoms such as fatigue, weight loss, and weakness.

NASH typically appears in people during middle age, in their 40s and 50s, but it can happen earlier or later in life. It occurs equally in men and women.

Early on, the buildup of fat does not affect the function of the liver, and you will have no symptoms. As fat continues to build up and inflammation occurs, liver function begins to decline and symptoms develop. This inflammation may lead to scarring and severe damage of the liver. There is no clear reason why some people with fatty liver develop NASH and others do not.

After the liver has been damaged, there is a much greater chance that the damage will continue and get worse. In some cases, the disease progression can stop and even reverse on its own without treatment. But in other cases NASH can slowly get worse and cause scarring (fibrosis) of the liver, which leads to cirrhosis. Cirrhosis means that the liver has become scarred and hardened and is not able to work normally.

NASH progresses to cirrhosis in about 15% to 20% of cases.²