Too much cholesterol in the blood raises the risk of heart disease. (Jose Leiva/Sun Journal/Associated Press)

Finally some good news about cholesterol and kids: A big U.S. government study shows that in the past decade, the proportion of children who have high cholesterol has fallen.

The results are surprising, given that the childhood obesity rate didn't budge.

How can that be?

Some experts think that while most kids may not be eating less or exercising more, they may be getting fewer trans fats. That's 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 Tuesday 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, 1 in 12 children ages 6 through 19 had high cholesterol. That was down from 1 in 9 during each of the earlier periods — roughly a 28 per cent decline.

The average overall cholesterol level fell from 165 to 160. In children, 200 is considered too high.

Cholesterol declines in adults

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.

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.

Cholesterol levels have been declining in adults, too. The incidence of high cholesterol dropped about 27 per cent in the last decade, from about 1 in 5 adults at the beginning of the period to 1 in 7 at the end.

But cholesterol-lowering drugs known as statins were a big part of the reason for that decline — millions of adults take them. Children are rarely given statins.

In 2010, Statistics Canada said blood tests of thousands of Canadians showed 41 per cent of adults had a high total cholesterol level.