For their new study, Dr Lisa Powell from the University of Illinois at Chicago and her co-author Binh Nguyen used data from nationally-representative health surveys conducted in the United States between 2003 and 2008.
On two different occasions, more than 9 000 teens were asked to recall everything they'd had to eat or drink in the past 24 hours. Parents were asked the same question for their younger children.
Between 24 and 42% of kids and teens had gotten take-out or eaten at a fast-food restaurant during each day they were questioned, and seven to 18% had eaten at a full-service restaurant.
How the study was done
Based on the researchers' calculations, adolescents ate and drank an extra 1302kJ on days they had fast food and an extra 1121kJ on days they ate at a full-service restaurant. Younger kids age two to 11 had an extra 529kJ and 672kJ on those days, on average.
Kids from poorer families got the most extra kilojoules on days when they went to a fast-food or sit-down restaurant.
"This is something that we really should be worried about, because this is going to increase health disparities among different socioeconomic groups," Dr Powell said.
Restaurants to play positive role
Eating at either type of restaurant was also tied to a drop in the amount of milk kids drank during the day, the researchers reported.
Their study was funded by the National Cancer Institute and the Centers for Disease Control and Prevention.
Dr Jason Block, an obesity and nutrition researcher from Harvard Medical School, said that although fast-food chains may be especially concerning because they advertise to kids, sit-down restaurants aren't necessarily healthier places to eat.
"There's been an assumption that fast-food meals are bad where full-service restaurants tend to get a pass," said Dr Block, who wasn't involved in the new research.
"Full-service restaurant meals are high-kilojoule, high-fat, high-sodium as well, and that should be a focus of people's interest, not just fast food."
"The restaurant industry is employing a wide range of strategies to play a positive role in food and healthy living issues, including advocating for a national nutrition information standard and adding more healthful items to menus," Joan Rector McGlockton, vice president of industry affairs and food policy at the National Restaurant Association,said.
"In fact, more than 110 restaurant brands representing 30 000 locations have committed to the National Restaurant Association's Kids LiveWell program, offering menu options that meet the 2010 USDA Dietary Guidelines."
(Reuters Health, November 2012)