A large number of young men and women who appear to be healthy and not overweight already have hardened blood vessels, putting them at risk of heart disease, a new study finds.

Canadian researchers studying normal weight and apparently healthy young people found that almost half already had atherosclerosis, a condition in which the arteries become clogged with fatty plaque deposits.

The condition, typically found in older adults with obesity, high cholesterol or diabetes, can eventually lead to heart disease or stroke. But this study, presented at the Canadian Cardiovascular Congress in Vancouver, found it in apparently healthy young adults too.

The researchers, led by Dr. Eric Larose, an interventional cardiologist at the Institut universitaire de cardiologie et de pneumologie de Québec, say their study is more evidence that young people are not immune to heart disease.

"They need to realize even if though they are young, apparently healthy and not obese, they can still develop premature atherosclerosis," Larose tells CTV News.

The study focused on 168 young adults between the ages of 18 and 35 who had no known heart disease risk factors, such as diabetes, smoking, high cholesterol, high blood pressure, or a family history of premature heart disease. None were clinically obese and all had a body mass index below 30.

The researchers measured the volunteers' waists. They also used magnetic resonance imaging (MRI) to measure various body fat deposits, including subcutaneous fat (fat under the skin that can be measured by pinching it with calipers) as well as intra-abdominal fat, which is also known as "visceral" fat. Finally, they used MRI to look for signs of atherosclerosis of the carotid arteries in the neck.

"What we found, which was very surprising, is that 48 per cent of these apparently healthy, young adults already had atherosclerosis that could be identified through magnetic resonance imaging," Larose said.

The MRI scans also showed hidden fat deposits inside many of these seemingly thin people. It's thought that this fat leads to the atherosclerosis.

"This visceral fat produces a plethora of toxic substances which will damage the vessel wall," says Larose.

As for why apparently healthy young adults are developing atherosclerosis, Larose suspects it's about unhealthy lifestyle habits, such as poor eating habits and a lack of exercise.

He says the study's findings are a reminder that even young adults who appear healthy by the usual measurements can still have atherosclerosis.

"So this is really a message of prevention where the younger adults need to be very careful about what they eat and how they exercise," he says.

The researchers add that while it's not feasible to screen all young adults for atherosclerosis with MRI imaging, they found that measuring waist circumference at the doctor's office is just as effective.

An enlarged waist circumference was predictive of increased visceral adiposity and of premature atherosclerosis. In fact, it was almost as precise as by MRI.

He added that it's unfortunate that more doctors are not making use of this simple measure.

"When was the last time that your physician measured your waist circumference? I can't remember the last time mine was measured, and it is actually the strongest predictor we found," he said.

With a report from CTV's Avis Favaro and producer Elizabeth St. Philip