The federal government proposed sweeping new guidelines on Thursday that could push the food industry to overhaul how it advertises cereal, soda pop, snacks, restaurant meals and other foods to children.

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Citing an epidemic of childhood obesity, regulators are taking aim at a range of tactics used to market foods high in sugar, fat or salt to children, including the use of cartoon characters like Toucan Sam, the brightly colored Froot Loops pitchman, who appears in television commercials and online games as well as on cereal boxes.

Regulators are asking food makers and restaurant companies to make a choice: make your products healthier or stop advertising them to youngsters.

“Toucan Sam can sell healthy food or junk food,” said Dale Kunkel, a communications professor at the University of Arizona who studies the marketing of children’s food. “This forces Toucan Sam to be associated with healthier products.”

The guidelines, released by the Federal Trade Commission, encompass a broad range of marketing efforts, including television and print ads, Web sites, online games that act as camouflaged advertisements, social media, product placements in movies, the use of movie characters in cross-promotions and fast-food children’s meals. The inclusion of digital media, such as product-based games, represents one of the government’s strongest efforts so far to address the extension of children’s advertising into the online world, which children’s health advocates say is a growing problem.

The guidelines are meant to be voluntary, but companies are likely to face heavy pressure to adopt them. Companies that choose to take part would have five to 10 years to bring their products and marketing into compliance.

“There’s clearly a demand hidden behind the velvet glove of the voluntary language,” said Dan Jaffe, an executive vice president of the Association of National Advertisers, a trade group that represents marketers like Kraft Foods and Campbell Soup.

By explicitly tying advertising to childhood obesity, the government is suggesting there is a darker side to cuddly figures like Cap’n Crunch, the Keebler elves, Ronald McDonald and the movie and television characters used to promote food. It also raises the question of whether they might ultimately share the fate of Joe Camel, the cartoon figure used to promote Camel cigarettes that was phased out amid allegations that it was meant to entice children to smoke.

“Our proposal really covers all forms of marketing to kids, and the product packaging and the images and themes on the cereal boxes have tremendous appeal to kids,” said Michelle K. Rusk, a lawyer with the trade commission. “The goal is to encourage children to eat more healthy foods because obesity is a huge health crisis.”

The F.T.C. said that in 2006, food companies spent nearly $2.3 billion to advertise to children.

The food industry immediately criticized the proposal, saying that it had already taken significant steps to improve recipes and change the way it advertises to children.

Kellogg, the company that makes Froot Loops, said in a statement that it would review the proposal and that it was committed to improving “the nutrition credentials” of its products. “We have very specific criteria, based on a broad review of scientific reports, that determine how and what products we market to children,” the company said. The company has already reduced sugar and added whole grains in many cereals.

Scott Faber, a vice president of the Grocery Manufacturers Association, a group that represents food makers, said that ads for packaged foods on television shows aimed at children 2 to 11 had dropped significantly since 2004, and that the ads more often showed healthier types of foods. He said companies had also changed many recipes to reduce salt, sugar and fat and add healthful ingredients like whole grains. “The rate of reformulation is going to increase, not as a result of the principles that were announced today but because consumers are demanding changes in the marketplace,” Mr. Faber said.

Many food companies participate in an industry-led effort, the Children’s Food and Beverage Advertising Initiative, to restrict some marketing activities. But each company that takes part is allowed to set its own nutritional criteria, which critics say undermine the program’s effectiveness.

Regulators said it was important for the entire industry to adhere to a uniform set of standards.

The guidelines were created at the request of Congress and written by the commission, the Food and Drug Administration, the Agriculture Department and the Centers for Disease Control. Regulators said they would take comments and consider changes before submitting a final report to Congress.

The guidelines call for foods that are advertised to children to meet two basic requirements. They would have to include certain healthful ingredients, like whole grains, fresh fruits and vegetables, or low-fat milk. And they could not contain unhealthful amounts of sugar, saturated fat, trans fat and salt.

The sugar requirement would limit cereals to eight grams of added sugar a serving, far less than many popular cereals have today. Froot Loops and Cap’n Crunch, for example, contain 12 grams of sugar a serving.

The salt restrictions are particularly stringent, and many packaged foods on the shelves today would have a hard time meeting them. In an initial phase-in period, the guidelines call for many foods to have no more than 210 milligrams of sodium a serving, while main dishes and meals, including both restaurant food and packaged food, could have no more than 450 milligrams. Today, a 15-ounce can of Chef Boyardee beef ravioli has two servings, with 750 milligrams of sodium per serving. The sodium restrictions would get tougher over time.

The federal agencies acknowledged that a “large percentage of food products currently in the marketplace would not meet the principles.”

The guidelines would apply to both young children and teenagers. The industry has said it should have greater leeway for teenagers, and Ms. Rusk said the agencies would consider those arguments.

Margo Wootan, director of nutrition policy for the Center for Science in the Public Interest, an advocacy group, predicted that the guidelines would force many companies to accept great restrictions and improve recipes.

“With all the concern about childhood obesity, I think there’s a lot of pressure on companies to do the right thing and follow these standards,” she said.

Jeffrey Chester, executive director for the Center for Digital Democracy, a group that focuses on Internet marketing to children, said the F.T.C. proposal had broader implications. “The youth obesity issue has placed all digital marketing in the regulatory cross hairs,” Mr. Chester said.

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