A new study finds that kids who spend the most time watching television or playing video games instead of exercising have narrower arteries in the back of their eyes -- an early indication that the kids are on the road to heart disease.

The study is the first of its kind, using changes in the arteries in the retina as a marker of blood-pressure changes that could signal heart trouble down the road.

The researchers from the University of Sydney looked at nearly 1,500 children six and seven years old. They had their parents answer a 193-item questionnaire, reporting the number of hours spent each week in indoor and outdoor physical activity, as well as on sedentary activity such as watching television, video games, computer time and reading.

The researchers then took digital photographs of the backs of children's eyes and calculated the average narrowing in the vessels.

They found that children who spent less than 30 minutes a day watching TV or playing computer games and were more physically active had normal blood vessels in their eyes. But those who spent more than an hour and a half in front of screens each day, and had the lowest level of activity, had narrowed arteries.

These sedentary kids had retinal arteries that were, on average, 2.2 microns narrower than those of the active kids. A micron is one thousandth of a millimetre.

Each hour per day of TV viewing time was associated on average with 1.53 microns narrower retinal arteriolar caliber.

The data appear in Arteriosclerosis, Thrombosis and Vascular Biology, a journal of the American Heart Association.

While the numbers sound small, microvascular caliber in the retina is a marker for heart disease and high blood pressure in adults.

"The arteries in the eye are part of the arteries in the body that put you at risk for stroke and heart attack in the long run," says Dr. Beth Abramson of the Heart and Stroke Foundation.

"In fact, older people who have narrower arteries in the back of their eyes probably have that because they have high blood pressure for a long time. So we are seeing young children with potential changes that could be detrimental in the long run."

After crunching the numbers, the researchers were able to calculate that each hour of screen time caused retinal artery changes that were similar to that associated with 10 millimeters of mercury (mm HG) increase in systolic blood pressure.

Those kinds of changes are typically seen in adults suffering from chronic high blood pressure. Yet some were as young as six.

Bamini Gopinath, lead author and senior research fellow at the Center for Vision Research at the University of Sydney says the changes his team witnessed suggest that "unhealthy lifestyle factors may influence microcirculation early in life and increase the risk of cardiovascular disease and hypertension later in life."

Gopinath says the changes in retinal arteries aren't caused by the long hours spent staring at TVs or computers; rather, it's an effect of sitting for long hours and not exercising.

Dr. Andrew Pipe of the University of Ottawa Heart Institute says the findings are concerning.

"Because we know that the changes we see in the cardiovascular system in childhood generally continue throughout growth into adulthood and so we may be seeing the very earliest signs of the damage that's produced by being physically inactive amongst our young," he told CTV News.

The researchers didn't continue to follow the children's health after the study ended so it's not known if changes to the kids' lifestyles led to changes in their retinal arteries. But Pipe believes it's a study with a warning.

"Send your children out to play and make sure your children are playing with baseballs and chasing hockey pucks and kicking soccer balls in parks, in laneways, in driveways," he advises.

Gopinath agrees, saying "free play" should be promoted at both home and school.

"Replacing one hour a day of screen time with physical activity could be effective in buffering the effects of sedentariness on the retinal microvasculature in children."

With a report from CTV medical specialist Avis Favaro and producer Elizabeth St. Philip