Children eat nearly two fifth more calories if they watch TV ads for junk food, even if they are full.
And some kids are genetically programmed to overeat when bombarded by the ads.
The study of nine and ten-year-olds discovered the influence of ad breaks.
Researchers at Dartmouth's Norris Cotton Cancer Centre, and the C. Everett Koop Institute, United States, tested how a gene previously linked to obesity, played a role in overeating amongst children exposed to 'food cues.'
In a randomised experiment study 172 children ate lunch until they were full then were immediately shown a 34-minute television show that aired either food or toy advertisements, during ordinary commercial breaks.
The youngsters given snacks during the show and researchers measured how much they tucked in to.
The children were also tested for genetic variations in the Fat Mass and Obesity Associated Gene, known as FTO.
Children who watched food adverts ate an average of 41 per cent more calories of a recently advertised food, than those who watched ads for toys.
Comparing the fat mass gene, children at the highest genetic risk were affected more than three times as much by the adverts.
Assistant Professor Diane Gilbert-Diamond said: "This study shows that children overeat in response to TV food ads even when they are not hungry.
"More importantly, some children are genetically prone to eat much more in response to those cues.
"The findings may help us understand how genes predispose people to obesity by amplifying the response to environmental food cues.
"If that finding is confirmed, limiting exposure to food advertising and other food cues would be key to combating child obesity."
Obesity is a known risk factor for cancer, diabetes and other health problems.
The study was published in the International Journal of Obesity.