OTTAWA - Watching TV at mealtime can make children overeat, as the distraction overrides signals that normally make a person feel full, Canadian research has found.

It's been known for years children and teens who watch a lot of TV tend to be heavier than those who don't. But one explanation was television watchers get less exercise, and probably swallow more junk as they sit than kids out throwing footballs or riding bicycles.

Now Harvey Anderson has another explanation - TV is somehow overriding basic signals to the brain that would tell the boys to stop eating.

"It seems that what mother and grandmother said was right, and we (scientists) have to prove the obvious," he said. "You should just turn the damn TV off."

To study the effect of television, the study team handed out unlimited free pizza.

Boys aged nine to 14 ate 22 per cent more pizza - an average of 228 calories' worth - while watching The Simpsons than when they sat in a quiet area without TV. In both cases, the boys were offered as much as they wanted, with fresh, hot pizza arriving every few minutes, and told to eat until they felt comfortably full.

The extra calories are almost equal to one slice of typical frozen pizza, which typically has about 300 calories, though in this experiment the boys got small pizzas cut into quarters, for more precise measuring.

As they ate second and third helpings, their bodies were churning out messages to say: "I'm full." One signal comes from a group of hormones secreted in the digestive system, which sends the "full" message to the brain. Another comes from the rising level of blood sugar as a person eats and digests.

But the TV watchers ignored both sets of signals.

And they ate just as much even after having a sugary drink half an hour before the free lunch. Normally a drink like that would cut their appetites.

"What's happening is more than the lack of physical activity," Anderson said. "It's the 'override' by the environment," which drowns out signals that people would notice if they were sitting around a kitchen table, minus the TV.

The study was published in Pediatric Research, a medical journal, and funded by the Canadian Institute of Health Research. Anderson teaches at the University of Toronto's 's department of nutritional sciences.

He feels the findings likely apply to adults as well. A recent study of women found they ate about 60 calories more if they were listened to an audio recording of a story.

"We can't just blame fast-food restaurants" for obesity, he says. "Surely we have some responsibility for the environments we put ourselves in. You don't go to the all-you-can-eat smorgasbord every night. You don't go to the bar and watch the football game while drinking beer and eating chicken wings all night."

In many ways, the science of nutrition is rediscovering some very old messages, he said. A healthy diet is one with fruit and vegetables, eaten in a family setting, without electronic distraction.

"It's just common sense," he said, "but common sense can be pretty uncommon."