The reason American children are overweight and obese is no mystery, says a new study commissioned by the Kaiser Family Foundation. Children are getting a steady diet of advertisements urging them to consume the worst types of food, the researchers say.

The study combines content analysis of TV ads with detailed data about children's viewing habits, to provide an estimate of the number and type of TV ads seen by children of various ages. The study found that tweens ages 8-12 see the most food ads on TV, an average of 21 ads a day, or more than 7,600 a year.

Teenagers see slightly fewer ads, at 17 a day, for a total of more than 6,000 a year. For a variety of reasons -- because they watch less TV overall, and more of their viewing is on networks that have limited or no advertising, such as PBS and Disney -- children ages 2-7 see the least number of food ads, at 12 food ads a day, or 4,400 a year.

For each age group studied, food was the top product seen advertised. Thirty-two percent of all ads seen by 2-7 year olds were for food, while 25 percent of ads seen by 8-12 year olds and 22 percent of ads seen by 13-17 year olds were for food. Of all genres on TV, shows specifically designed for children under 12 have the highest proportion of food advertising (50 percent of all ad time).

"Children of all ages see thousands of food ads a year, but tweens see more than any other age group," said Vicky Rideout, vice president and director of the Program for the Study of Entertainment Media and Health at the Kaiser Family Foundation. "Since tweens are at an age where they're just becoming independent consumers, understanding what type of advertising they are exposed to is especially important."

Types of Food Advertised Of all food ads in the study that target children or teens, 34 percent are for candy and snacks, 28 percent are for cereal, and 10 percent are for fast foods. Four percent are for dairy products and 1% for fruit juices. Of the 8,854 ads reviewed in the study, there were none for fruits or vegetables targeting children or teens.

Appeals One in five food ads targeting children or teens include a push to a website, and a similar proportion include the offer of a premium, such as a game or toy. About one in ten have a tie-in to a children's TV or movie character.

Physical Activity Portrayed Fifteen percent of all food ads targeting children or teens include depictions of a physically active lifestyle, such as showing children skateboarding, snowboarding, or playing basketball.

Public Service Advertising The study also measured children's exposure to public service messages on fitness or nutrition (whether donated or paid). Children 2-7 and 8-12 see an average of one such message every 2-3 days (164 a year for 2-7 year-olds and 158 a year for 8-12 year-olds). Teens 13-17 see just one such message per week, for an average of 47 per year.

"While public service ads on fitness and nutrition may well play an important role in helping to fight childhood obesity, we need to be realistic about our expectations, given how few such messages children see," Rideout said.