WASHINGTON (Reuters) – Young American adults who learn they have a low immediate risk of heart disease may be making a mistake if they sigh with relief and relax – their lifetime risk could be high, doctors cautioned this week.

A new analysis of heart disease risk studies shows that about half of people under the age of 50 who appeared to have a low risk of heart disease for the next 10 years already had damage to the arteries that could cause trouble later.

"We found that about half of individuals who are 50 years of age or younger and at low short-term risk for heart disease may not remain at low risk throughout their lives," said Dr. Jarett Berry of the University of Texas Southwestern Medical School, who helped lead the study.

Usually, a doctor assesses a patient's risk of heart disease using the Framingham Risk Score, which takes into account cholesterol levels, blood pressure and other factors to predict cardiovascular risk for the next 10 years.

And, almost automatically, anyone under the age of 50 has a low 10-year risk.

But heart disease usually develops slowly, as "plaque" builds up in arteries, reducing blood flow and causing unstable clumps that can break off and cause strokes or heart attacks.

Berry's team looked at the 10-year and lifetime risk scores for nearly 4,000 people age 50 and younger taking part in two clinical studies.

They found that 91 percent of those 50 and younger had a low immediate risk of heart disease and would have been told as such by their doctors. But half had a high lifetime risk, they reported in the journal Circulation.

The volunteers had undergone unusually thorough examinations because they were in the studies, so Berry and colleagues looked at the ultrasound measurements of their carotid arteries and CAT scans for calcium. Both types of scans can show early evidence of artery disease. They showed that the people who had a high lifetime risk according to cholesterol, blood pressure and other measures also had the beginnings of physical evidence of heart disease – thicker artery linings and hardened plaques.

"What we found was there were significant differences in the presence and progress of atherosclerosis," Berry said in a telephone interview.

Usually, doctors just tell patients about their 10-year risk, but if patients knew their lifetime risks they could act right away to change their lifestyles, Berry said.

Cardiovascular disease, which includes heart attack, stroke and heart failure, is the number one killer of people in the United States, killing as many people each year as cancer, chronic lower respiratory diseases, accidents and diabetes combined, the American Heart Association says.