TRENTON, N.J. — In a new and disturbing twist on the obesity epidemic, some overweight teenagers have severe liver damage caused by too much body fat, and a handful have needed liver transplants.

Many more may need a new liver by their 30s or 40s, say experts, who warn that pediatricians need to be more vigilant. The condition, which can lead to cirrhosis and liver failure or liver cancer, is being seen in kids in the United States, Europe, Australia and even some developing countries, according to a surge of recent medical studies and interviews with doctors.

The American Liver Foundation and other experts estimate 2 percent to 5 percent of American children over age 5, nearly all of them obese or overweight, have the condition, called nonalcoholic fatty liver disease.

"It's clearly the most common cause of liver disease," said Dr. Ronald Sokol, head of public policy at the liver foundation and a liver specialist at Children's Hospital and University of Colorado Denver.

Some experts think as many as 10 percent of all children and half of those who are obese may suffer from it, but note that few are given the simple blood test that can signal its presence. A biopsy is the only sure way to diagnose this disease.

As fat builds up, the liver can become inflamed and then scarred over time, leading to cirrhosis, a serious condition, which in years past was mostly caused by hepatitis or drinking too much alcohol. Liver failure or liver cancer can follow, but if cirrhosis has not yet developed, fatty liver disease can be reversed through weight loss.

The disease is most common in overweight children with belly fat and certain warning signs, such as diabetes or cholesterol or heart problems. However, it's been seen in a few children of normal weight.

Genetics, diet and exercise level all play a role. It is most prevalent among Hispanics, relatively rare among African-Americans, and more common among boys than girls.

"There are people in their 30s or early 40s that will require a liver transplant" from developing the condition as a kid, predicts Dr. José Derdoy, head of liver transplants at Cardinal Glennon Children's Medical Center in St. Louis. He's treated a 15-year-old 530-pound boy and many others with the condition.

Experts blame obesity, with about two-thirds of all Americans overweight. With fatty liver disease becoming more common in adults, many experts predict it will become the top cause of liver transplants by 2020.

"There aren't enough livers to go around," said Dr. Philip Rosenthal of the University of California-San Francisco Children's Hospital.

Nonalcoholic fatty liver disease often has no early symptoms in children or adults, but a fat belly is one signal. And diabetes, high cholesterol, high triglycerides or heart problems often accompany the disease.

As fatty liver disease worsens, these symptoms can appear: