That figure is an estimate based on findings from a San Diego study published in Pediatrics.
Nonalcoholic fatty liver disease is fat in the liver without liver inflammation and liver damage.
It can worsen into nonalcoholic steatohepatitis (NASH), which is fatty liver with liver inflammation and liver damage.
NASH can eventually cause permanent liver scarring () and may even require a liver transplant.
In short, fat in the liver can be a slippery slope toward serious liver problems.
Fatty Liver Study
The new study comes from researchers including Jeffrey Schwimmer, MD, of the pediatrics department at the University of California, San Diego.
They analyzed liver samples taken from autopsies performed on 742 San Diego children from 1993 to 2003.
The children were 2-19 years old when they died in accidents, homicides, or suicides. None were in the hospital or using drugs (including prescription drugs) when they died.
Most of the kids (44%) were white, followed by Hispanics (34%), blacks (11%), Asians (9%), and other ethnic backgrounds (2%). Nearly three-quarters were boys.
Most of the kids -- 72% -- had fat-free liver samples. About 15% had a relatively small amount of fat in their liver sample.
But 13% of the children had enough fat in their liver sample to indicate nonalcoholic fatty liver disease. Those kids' liver samples were at least 5% fat, the study shows.
Older children, Hispanics, and kids with a high BMI (body mass index) were the most likely kids in the study to have nonalcoholic fatty liver disease.
For instance, "overweight and obese children accounted for 81% of all of the cases of fatty liver," Schwimmer's team notes.
The researchers add that they found racial and ethnic differences, with the highest rate of nonalcoholic fatty liver disease among Hispanic kids and the lowest rate among black children.
"We estimate that the prevalence of fatty liver is 9.6% in children age 2 to 19 years in the county of San Diego," write Schwimmer and colleagues.
"If the prevalence is similar for the entire United States, this would represent [more than] 6.5 million children and adolescents," they write.
Schwimmer's team admits that they're not certain that the children in their study represent other kids. They also say they're not sure what caused the kids' fatty livers.
However, they conclude that "fatty liver is the most common form of pediatric liver disease," and that "is a major risk factor."
The researchers call for "effective prevention and treatment of obesity" in children to help curb kids' fatty livers.
"We must also consider secondary prevention strategies targeted toward preventing the development of fatty liver in children who are overweight," they write.
Overweight kids who are 5-9 years old may be at a good age for such prevention strategies, the researchers note.