Toddlers who spend more time lounging in front of the tube maintain less healthy diets, a Harvard Medical School (HMS) study announced yesterday.

Each additional hour of television viewed by three-year-olds translated into 46 more calories consumed each day.

“Obesity is very hard to treat once it has started,” said Sonia A. Miller, the HMS student who led the study. “We can already see the influence television has on three-year-old children, and it seems that reducing screen time will be very important in relation to the prevention of obesity.”

The study attributed toddlers’ weight gains to the consumption of fattier foods rather than lack of exercise. Many of the food items were high in calories, sugar, and fats, including trans fats.

The researchers also linked increased TV viewing to a lower consumption of healthier foods, such as fruits and vegetables.

Miller’s team said the fattier diets may have resulted from snacking while watching TV, or from the influence of commercials for unhealthy foods.

The report was unveiled at the American Heart Association’s Annual Conference on Cardiovascular Disease Epidemiology and Prevention in Orlando, Fla.

The study was based on questionnaires completed by 1,200 mothers about the television patterns of their three-year-old children. The researchers controlled their data for certain socioeconomic factors and the body mass index of the parents.

Miller cited the recommendation of the American Academy of Pediatrics, which suggests that children over the age of two watch a maximum of two hours of TV daily.

“We hope that our results will give people some understanding as to why limiting television time makes sense,” Miller said. “Hopefully, [the study] allows for parents to understand as well, not just clinicians and policy makers.”

Professor of Medicine Bruce Ryan Bistrian, who is also chief of clinical nutrition at Beth Israel Deaconess Medical Center, noted the difficulty of isolating a specific cause for the obesity epidemic.

“Although [the study] would suggest very strongly that the children are eating more and eating the wrong kinds of foods, they’re also doing less,” Bistrian said. “You don’t know if these associations are causative. It could be that these children who are watching TV are already overweight.”