Watching too much television at age 2 can translate into poorer eating habits in adolescence and poorer performance in school, researchers at Université de Montréal's School of Psychoeducation have found.

In a new longitudinal study published in Preventive Medicine, graduate student Isabelle Simonato and her supervisor, Professor Linda Pagani, looked at a birth cohort of nearly 2,000 Quebec boys and girls born between spring 1997 and 1998.

The children were followed since they were 5 months old as part of the Quebec Longitudinal Study of Child Development. When they reached 2 years of age, their parents reported on their daily television habits. Then, at age 13, the youths themselves reported on their dietary habits and behaviour in school.

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

Added Simonato: "Watching TV is mentally and physically sedentary behavior because it does not require sustained effort. We hypothesized 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."

In their study, the researchers found that every hourly increase in toddlers' TV viewing forecasted bad eating habits down the road -- an increase of 8 per cent at age 13 for every hourly increase at age 2. In questionnaires, those early-TV adolescents reported consuming more French fries, prepared meats and cold cuts, white bread, regular and diet soft drinks, fruit-flavoured drinks, sports drinks, energy drinks, salty or sweet snacks, and desserts.

Early TV viewing also translated into less eating of breakfast on school days (by 10 per cent) and led to more overall screen time at age 13. Every hour increase of TV also predicted a higher body mass index (a 10-per-cent increase) and less effortful behavior at school in the first year of secondary school, ultimately affecting performance and ambition.

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

The researchers also measured their results against revised screen time guidelines by the American Academy of Pediatrics, which reduced the amount of daily viewing from two hours a day to one a day for children between ages 2 and 5. Compared to children who viewed less than one hour a day at age 2, those who viewed between one and four a day later reported (at age 13) having less healthy dietary habits, skipping breakfast on weekdays, having a higher BMI, engaging in more intense screen time, and being less engaged as students.

"Because we had a lot of information on each child and family we were able to eliminate other psychological and socio-demographic factors that could have explained the results, which is a really ideal situation," said Simonato. "We even removed any influence of screen time habits at age 13 to really isolate long-term associations with toddler viewing."

Added Pagani: "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.

"Rewarding distraction and low mental effort via 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."