The results are surprising, given that the childhood obesity rate did not budge. Some experts think that while most kids may not be eating less or exercising more, they may be getting fewer trans fats. That is because the artery-clogging ingredient has been removed or reduced in many processed or fried foods such as doughnuts, cookies and french fries.

"That's my leading theory," said Dr. Sarah de Ferranti, director of preventive cardiology at Boston Children's Hospital. She wrote an editorial that accompanies the study.

The study did not look at the reasons for the decline, but its lead author, Dr. Brian Kit of the Centers for Disease Control and Prevention, said the theory makes sense.

The research, released online by the Journal of the American Medical Association, also showed that children's average overall cholesterol levels declined slightly.

Too much cholesterol in the blood raises the risk of heart disease. It isn't usually an immediate threat for most children, but those who have the problem often grow into adults with a high risk.

Kit and his colleagues drew data from an intensive national study that interviews people and does blood-cholesterol tests. They focused on more than 16,000 children and adolescents over three periods -- 1988-94, 1999-2002 and 2007-10.

During the most recent period studied, the number of children ages 6 through 19 with high cholesterol was 1 in 12. That was down from 1 in 9 in each of the earlier periods. The average overall cholesterol level fell from 165 to 160. In children, 200 is considered too high.

The study was the first in almost 20 years to show such a decline. Kids' cholesterol levels also fell between the 1960s and the early 1990s, probably because people were eating less fat and saturated fat. Adult cholesterol levels fell then, too.

The researchers in the latest study detected modest improvements in children's levels of so-called good cholesterol, which can protect the heart. That may be partly due to declines in teen smoking and childhood exposure to secondhand smoke over the last decade. Studies have found that chemicals in cigarette smoke can lower good cholesterol.

The bigger news was what happened with bad cholesterol and triglycerides. They went down by small but significant amounts.

In adults, when bad cholesterol levels drop, it's often because patients are using cholesterol-lowering drugs known as statins. But children are rarely given statins.

Last year, a government-appointed panel urged widespread cholesterol screening for children. It was controversial because of concerns it would lead to more kids being given medicine. Experts say statins should be used in only the worst cases -- less than 1%.

Artificial trans fats are known to decrease good cholesterol and increase bad cholesterol. In 2006, the federal government began requiring that packaged foods list the amount of trans fat per serving, a boon for careful shoppers.

Meanwhile, a push to take trans fats out of foods gained momentum. New York City banned artificial trans fats in restaurant food in 2008. California in 2010 became the first state to adopt such a ban. Even Crisco, the goopy shortening that was trans fat incarnate, was reformulated to take it out.

"I love the idea that reduced use of hydrogenated trans fats might be responsible" for the new study's results, Marion Nestle, a New York University professor of nutrition, food studies and public health, said in an email. "If so -- and as usual it's clear that more research is needed -- it would mean that public health measures like the trans fat ban in New York City are actually doing enough good to be measureable."

This is not the first study to suggest a payoff in trans fat policy efforts. The U.S. Department of Agriculture found that from 2005 to 2010, the average trans fat content in bakery items and other foods declined steeply. A small, preliminary CDC study published earlier this year found significant drops in trans fats in white adults between 2000 and 2009.

Despite the good news, experts remain worried.

Seventeen percent of U.S. children are obese, perhaps because they are still eating lots of carbohydrates and sugar. That, along with little exercise, can lead to diabetes and heart disease.

"We may have a small effect in the right direction from lower cholesterol, but I'm worried it will be overwhelmed by the earlier onset of obesity in younger and younger children," de Ferranti said. "I'm still pretty worried about how many kids are going to wind up patients of adult cardiologists."