Children who watch more TV between the ages of two and four may increase their risk of having a large waist size and weaker muscular fitness by the end of grade 4, Quebec research suggests.

To examine the association between hours spent watching TV and physical fitness, researchers from the University of Montreal assessed TV time, muscle strength and abdominal fat based on the standing long jump fitness test and waist circumference measurements in the second and fourth grade.

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Children over the age of two should not watch more than two hours of television per day, pediatricians say. (Billion Lim/AFP/Getty)

Parents of the 1,314 Quebec children reported how many hours of television the kids watched when they were 2.5 and 4.5 years of age. Trained examiners took waist measurements and gave the fitness tests.

The average TV time was 8.8 hours a week when the study began, which increased on average by six hours over the next two years to reach 14.8 hours a week by the time the children were kindergarten age.

Close to 15 per cent of the children watched more than 18 hours of TV a week, the researchers found.

Children over the age of two shouldn't watch more than two hours of television per day and those under two are discouraged from watching, according to the American Academy of Pediatrics.

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Each weekly hour of TV at 29 months of age corresponded to a decrease of about a third of a centimetre in the distance a child was able to jump.

The findings mean that a child who watches 18 hours of television at 4.5 years of age will by the age of 10 have an extra 7.6 millimetres of waist because of his or her habits.

"Our findings suggest that early childhood television viewing may also undermine future explosive leg strength and contribute to the accumulation of abdominal fat," lead author Dr. Caroline Fitzpatrick and her co-authors concluded in the International Journal of Behavioral Nutrition and  Physical Activity.

Sedentary TV time

The standing long jump is an indicator of health and points to someone's athletic ability for activities like skating and sprinting or sports such as basketball and soccer that require explosive leg strength, Fitzpatrick said.

She called childhood a critical period for the development of habits and preferred activities.

Watching more TV displaces more educational and physically active pursuits and risks teaching children inaccurate information about proper eating, said the study’s senior author Dr. Linda Pagani.

Snacking in front of the tube could also contribute while TV ads for fast food may sway kids' preferences towards sugary foods and saturated fats, the researchers said.

In the study, factors such as maternal height and weight, education and immigration status were taken into account.

As an observational study, no cause-and-effect relationship can be drawn. Parents may have underreported how much TV their children watched, the researchers noted.

They suggested randomly assigning children to programs designed to reduce sedentary behaviours like TV time to examine the long-term effects on fitness.