The new advertising campaign began Monday to help New Yorkers make the most of the city's calorie-posting rules -- by law, chain restaurants now must post food item calorie counts on menus and menu boards.

The five ads appearing in subway cars are designed to help people see how quickly fast-food calories add up and drive home the message that some foods can have deceptively high calorie counts.

Cathy Nonas, director of the health department's physical activity and nutrition program, said many people are unaware a typical adult needs only 2,000 calories a day to maintain a steady weight.

One poster shows an apple raisin muffin that looks harmless, healthy even, but has 470 calories -- nearly one-quarter of an adult's daily allowance.

"If you're eating it as a snack, you may want to split it with a friend," Nonas, a registered dietitian, said in a statement.

"Many people end up overweight just by going with the flow. Now that this information is available in chain restaurants, it's easy to make healthier choices."