None of them had been diagnosed with heart disease before deployment, researchers said.

"This is a young, healthy, fit group," said the study's lead author, Dr. Bryant Webber, from the Uniformed Services University of the Health Sciences in Bethesda, Maryland.

"These are people who are asymptomatic, they feel fine, they're deployed into combat," he told Reuters Health.

"It just proves again the point that we know that this is a clinically silent disease, meaning people can go years without being diagnosed, having no signs or symptoms of the disease."

Webber said the findings also show that although the U.S. has made progress in lowering the nationwide prevalence of heart disease, there's more work that can be done to encourage people to adopt a healthy lifestyle and reduce their risks.

Heart disease accounts for about one in four deaths - or about 600,000 Americans each year, according to the Centers for Disease Control and Prevention.

The new data come from autopsies done on U.S. service members who died in October 2001 through August 2011 during combat or from unintentional injuries. Those autopsies were originally performed to provide a full account to service members' families of how they died.

The study mirrors autopsy research on Korean and Vietnam war veterans, which found signs of heart disease in as many as three-quarters of deceased service members at the time.

"Earlier autopsy studies... were critical pieces of information that alerted the medical community to the lurking burden of coronary disease in our young people," said Dr. Daniel Levy, director of the Framingham Heart Study and a senior investigator with the National Institutes of Health.

The findings are not directly comparable, in part because there was a draft in place during the earlier wars but not for Operations Enduring Freedom and Iraqi Freedom/New Dawn. When service is optional, healthier people might be more likely to sign up, researchers explained.

Still, Levy said the new study likely reflects declines in heart disease in the U.S. in general over that span.

Altogether the researchers had information on 3,832 service members who'd been killed at an average age of 26. Close to 9 percent had any buildup in their coronary arteries, according to the autopsies. And about a quarter of the soldiers with buildup in their arteries had severe blockage.

Service members who had been obese or had high cholesterol or high blood pressure when they entered the military were especially likely to have plaque buildup, Webber and his colleagues reported Tuesday in the Journal of the American Medical Association.

More than 98 percent of the service members included were men.

"This study bodes well for a lower burden of disease lurking in young people," Levy, who wrote an editorial published with the report, told Reuters Health.

"Young, healthy people are likely to have a lower burden of disease today than their parents or grandparents had decades ago."

That's likely due, in part, to better control of blood pressure and cholesterol and lower rates of smoking in today's service members - as well as the country in general, researchers said.

However, two risks for heart disease that haven't declined are obesity and diabetes, which are closely linked.

"Obesity is the one that has not trended in the right direction," Levy said.

"Those changes in obesity and diabetes threaten to reverse some of the dramatic improvements that we are seeing in heart disease death rates," he added.

SOURCE: bit.ly/JjFzqx Journal of the American Medical Association, online December 25, 2012.

• A Permanent Economic Emergency