Too much television at the age of two appears to have a big impact on health during adolescence, the results of a new study suggest.

According to the findings, children who watch more than an hour of television every day at the age of two are less likely to eat healthy foods and perform well in school by the age of 13.

Canadian researchers followed the progress of almost 2,000 boys and girls from when they were five months old until they were 13 years. During that time, dietary habits, behaviour at school and television habits were assessed.

All of the children were born in the late 1990s.

"Not much is known about how excessive screen exposure in early childhood relates to lifestyle choices in adolescence. This birth cohort is ideal because the children were born before smartphones and tablets, and before any paediatric viewing guidelines were publicised for parents to follow.

"They were raising their children with TV and seeing it as harmless. This makes our study very naturalistic, with no outside guidelines or interference - a huge advantage," explained the study's supervisor, Prof Linda Pagani, of Université de Montréal.

Watching television is considered a physically and mentally sedentary behaviour, so the researchers suggested this could have a major impact on toddlers.

"We hypothesised that when toddlers watch too much TV it encourages them to be sedentary, and if they learn to prefer effortless leisure activities at a very young age, they likely won't think much of non-leisure ones, like school, when they're older," they noted.

The study found that children who watched too much television at the age of two were more likely to have bad eating habits at the age of 13 (an increase of 8% at age 13 for every hourly increase at the age of two).

These bad habits included consuming more salty and sweet snacks, energy drinks, fizzy drinks, processed meats and French fries.

Those who watched a lot of TV at the age of two were also less likely to eat breakfast at the age of 13 and were more likely to have a higher body mass index (BMI).

They also tended to make less effort in school, which affected their overall performance and ambition.

"This study tells us that overindulgent lifestyle habits begin in early childhood and seem to persist throughout the life course. An effortless existence creates health risks. For our society, that means a bigger healthcare burden associated with obesity and lack of cardiovascular fitness," Prof Pagani said.

The researchers measured their results against revised screen time guidelines issued by the American Academy of Pediatrics (AAP). These guidelines reduced the amount of daily viewing from two hours per day to one hour, for children aged between two and five years.

Overall, two-year-old children who watched TV for less than one hour per day compared to those who watched it for between one and four hours per day had a healthier diet at the age of 13, were more likely to eat breakfast, had a lower BMI, watched less TV and were more engaged in school.

"In preschool, parents use screen time as a reward and as a distraction. They establish quiet ‘idling' at a teachable moment when children could actually be learning self-control. Using distraction as a reward to help children behave in situations where they should be learning self-control sets them on a trajectory where they will seek out distraction when faced with demands for cognitive effort," Prof Pagani explained.

She added that rewarding distraction and low mental effort with entertainment ‘will later influence a young person's commitment to school and perseverance in their studies'.

"So we believe the AAP guidelines of not more than one hour of TV viewing for young children is correct, to ensure healthy developmental trajectories in adolescence."

Details of these findings are published in the journal, Preventive Medicine.