Replacing one hour a day of kids' screen time with physical activity could help buffer the effects of sedentariness, researchers say. Darrin Zammit Lupi/Reuters

Young children who spend a lot of time in front of a TV or computer screen show changes in the back of their eyes that may point to future heart disease risk.

Among the nearly 1,500 children aged six and seven in the Australian study, those who spent more time running around and playing outside had the healthiest blood vessels in their retinas. But the researchers found narrowed vessels in children who spent the most time sitting in front of a TV or playing video games.

For each hour of viewing, a vessel in the retina narrowed by an average of 2.3 microns — similar to that of a 10 millimeters of mercury increase in systolic blood pressure, the top number, in the children, Bamini Gopinath of the Center for Vision Research at the University of Sydney and his colleagues said in Wednesday's issue of Arteriosclerosis, Thrombosis and Vascular Biology: Journal of the American Heart Association.

Previous studies in both adults and children show that narrower "retinal arteriolar calibers" are markers of cardiovascular disease risk factors such as obesity and high blood pressure. The influence of sedentary behaviour on kids' retinas hadn't been studied until now, the team said.

"Excessive screen time leads to less physical activity, unhealthy dietary habits and weight gain," Gopinath said. "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. Free play should be promoted and schools should have a mandatory two hours a week in physical activity for children," Gopinath suggested.

The researchers concluded that children showing narrowed vessels may need to be monitored more closely.

For the study, parents answered a questionnaire about the number of hours spent each week in indoor and outdoor physical activity and sedentary activity such as watching television, videogames, computer time and reading.

Researchers took digital photographs of the back of each child's eye, then calculated the average narrowing in the vessels. Height, weight, body mass index and three separate blood pressure measurements were taken and averaged.

On average, the children spent 1.9 hours per day in screen time and 36 minutes a day in total physical activity.

The researchers suspect that exercise helps how arteries function by increasing blood flow, but this idea hasn't been tested in children.