How many calories would you like with those fries?

Would it surprise you to know that a crispy chicken salad at Burger King has as many calories (670) as a Whopper? Or that a 16-ounce mocha at Starbucks has twice the calories of a cappuccino?

These numbers aren't exactly trade secrets -- you can find them on the restaurants' websites -- but in the next few years, they will start showing up on their menus as well, thanks to a small provision in the massive health care law President Obama signed last week.

The new rule, which applies to restaurant chains with 20 or more locations, won't formally kick in for at least a year. But consumer advocates say it could be a powerful weapon in the battle against obesity, simply by allowing consumers to see how many calories they're eating when they dine out.

"Right now, when coffee drinks can range from 20 calories to 800 calories and even hamburgers can range from 250 calories to over 1,000, people have no idea what they're eating," said Mary Story, a nutritionist and obesity expert at the University of Minnesota. "I try not to eat out. It's just too easy to gain weight."

Where "menu labeling" is in effect, there's already evidence that it's having an effect. In New York City, which passed the first such law in 2008, restaurants have trimmed the calorie content of some of their most popular items, and consumers have shaved off some of the calories they order at each sitting.

Some restaurant chains, too, have found ways to lighten their fare. McDonald's, for example, shaved 70 calories from a large order of fries, and Dunkin' Donuts cut 130 calories from a glazed donut.

Even the restaurant industry, which initially fought the idea, has endorsed the new menu requirement.

'I know how bad it is'

Consumers, though, seem to have mixed reactions.

Last week, Greg Lundin of Champlin wasn't thinking about calories as he stared at McDonalds' menu board at the Mall of America's food court.

"I'm just thinking about the money, how much it costs," said the 20-year-old. But he wouldn't mind knowing the cost in calories, too, he said. "If I felt fat that day, then yes, it would probably affect me."

His friend Jenny Ziemer, 20, of St. Paul, seemed less interested. "I don't know that it would affect me," she said, "because I know how bad it is."

Asked if she would order less if the calories were posted, she said, "I know I would."

George Masmanides, 39, of Savage, called it a great idea. "Some days you want a bacon cheeseburger, [but] having the information right there is handy," he said. Added his wife, Patti: "It could make you think twice."

But 19-year-old Devyn Johnson, a student at the University of Minnesota, seemed to resent government intrusion in the calorie wars. "If you really want to know, you can find it online," she said, as she and three friends munched on pretzel dogs (340 calories) at Auntie Anne's. "I don't think it's our country's responsibility to be like, 'Oh, you need to get skinny and watch your calories'." Besides, she added, "It kind of takes the fun out of food."

Consumer advocate Margo Wootan, who campaigned for the new rule, shrugs off the nanny-state accusations.

"We're not trying to tell anybody what to eat," said Wootan, director of nutrition policy for the Center for Science in the Public Interest in Washington. "We're just trying to ensure that information is available."

On average, Wootan notes, Americans eat out four meals a week and consume a third of their calories dining out. In the war against obesity, that makes this a key battlefield.

"There are few other places where, in a split-second decision without much sacrifice, you can cut hundreds, even 1,000 calories, from your diet," she said.

In practice, the results aren't always that dramatic. In 2009, a year after New York City's law took effect, only about half of lunch-hour customers said they noticed the calorie count on menus, and only 15 percent used it to make a decision. Those who did, however, consumed an average of 754 calories, about 106 fewer than those who didn't, a survey showed.

A study by Stanford University also found that, from 2008 to 2009, calorie consumption dropped by 6 percent among Starbucks customers in New York City. "That's terrific," says Wootan, "when you get that kind of an impact from just going to a coffee shop."

There's also a sense that consumers are becoming more health-conscious, said David Siegel, executive director of the Minnesota Restaurant Association.

Last week, Panera became the first chain to start posting calories on its menus nationally, including in Minnesota; more are expected to follow.

Many chains might tweak their menus, but they will do it selectively, said Dennis Lombardi, an industry consultant with WD Partners in Ohio. "Don't expect the Whopper to be reformulated," he said. But salads with fattening dressings might get an overhaul. And new low-calorie products might make their debut to try to balance the menu offerings, he said.

"People are still going to eat out," Wootan said. "They're just, hopefully, going to eat a little differently."

Staff writer Mike Hughlett contributed to this article. Maura Lerner • 612-673-7384