Chicago ? When Susan Connor’s 3-year-old son started humming the McDonald’s jingle, a research project was born.

Connor knew where he’d heard the fast food giant’s catchy tune – on the Disney Channel during “The Wiggles,” a show for preschoolers.

“He had absorbed that from watching TV,” said Connor, whose study on food ads aimed at toddlers appears in the October issue of Pediatrics. “It would be a marketer’s dream to know they were that successful.”

Messages for high-fat, high-sugar foods permeate programming for preschoolers on Nickelodeon, the study found. On the Disney Channel’s shows for the youngest children and even on Public Broadcasting Service shows such as “Sesame Street,” companies woo tots’ loyalty by linking logos, licensed characters and slogans with fun and happiness.

Disney and PBS promote themselves as ad-free, but fast food companies dominated sponsor messages during programming for toddlers, Connor found, making up 82 percent of sponsor messages on PBS preschool programming and 36 percent of messages on Disney’s toddler block of shows.

Advocates said the study adds to mounting evidence that food marketers are trying to hook the youngest children as lifelong customers. Promotions go both ways with TV characters from children’s shows used on the packaging of sugary cereals, fruit-flavored snacks and other foods.

Last week, the Federal Communications Commission announced plans to study links between the ads, viewing habits and the rise of childhood obesity. For now, marketing of food to children is unregulated.

Previous studies have found that kids as young as 3 who see TV ads are more likely to request and eat advertised foods high in fat, sodium and sugar. American children from infancy to age 6 watch an average of one hour of TV daily, and 8- to-18-year-olds watch an average of three hours daily. They see roughly 40,000 TV ads a year.

“It’s very concerning when childhood obesity is a major public health problem that preschool programs are still being sponsored by fast food restaurants and food that’s not healthy for children,” said Susan Linn of Harvard Medical School and a co-founder of the Campaign for a Commercial-Free Childhood. She was not involved in the study.