Swati Kapoor, 25, was about to order a double chocolate cake doughnut when she noticed something new on the rack at Dunkin' Donuts. A tag said 290 calories. In an instant, she switched to a chocolate frosted doughnut (230 calories).

"To prevent obesity," the skinny medical student explained, munching away at a table in 30th Street Station.

Philadelphia begins phasing in enforcement of its strictest-in-the-nation menu-labeling law tomorrow. This first part, requiring chain restaurants to list calories on food tags and menu boards, is a relatively simple proposition that research shows can influence ordering habits.

A similar law will take effect in New Jersey next year, and dozens of such bills are pending around the country, including in Harrisburg.

What's different in Philadelphia will become apparent on April 1, when restaurants with individual menus must list saturated fats, trans fats, carbohydrates, and sodium, in addition to calories, with every item.

No one really knows what will come of this broader experiment in attempted behavioral change.

"The majority of people, I believe, will see this as cumbersome and an overreaction and not necessary," said George McKerrow Jr., president and chief executive officer of Ted's Montana Grill, who anticipates having to expand the menu at his South Broad Street location from two pages to six.

Still, just two months after Ted's added calories alone to its menu here, responding to a New York City requirement, McKerrow has noticed a small but measurable change in Philadelphia: "Some people have chosen to eat the healthier items more often."

Restaurants initially fought all efforts to mandate labels on menus. As the movement spread, with dozens of variations proposed across the country, the industry switched its goal to uniformity: calories, yes; sodium, no.

It has won that fight everywhere except Philadelphia. City Council approved the measure in 2008, after viewing data that showed the impact of chronic diseases related to diet - diabetes is diagnosed in 13 percent of residents, high blood pressure in 36 percent - broken down by district.

Diabetics must manage their intake of carbohydrates (including sugar); too much sodium can raise blood pressure. Both are listed on the familiar nutrition-facts label on all prepackaged goods.

"But it is really hard for people, if they eat out, to know about the sodium content," city Health Commissioner Donald Schwarz said.

At Olive Garden, for example, nothing on the dinner menu hints at a difference between linguine alla marinara (900 milligrams of sodium, according to its Web site) and pork Milanese (3,100 mg) - or notes that the Food and Drug Administration recommends less than 2,300 mg a day total, a line that must be added by April 1.

"It would make a difference," said Nashikai Ianscoli, 57, of Center City, who has had to go on a diet to control her blood pressure. She grew up on a farm in the South where her mother got fresh vegetables by the bushel.

Much has changed since she was a child.

"Back in the 1970s, eating out was a special occasion. What people ate didn't matter as much," said Margo G. Wootan, nutrition-policy director at the Center for Science in the Public Interest.

Americans now get an estimated one-third of their calories from meals outside the home. And though FDA serving sizes haven't changed, restaurant portions, especially fast food, have doubled or tripled. Skyrocketing obesity rates - one-third of Americans are obese, about the same as in Philadelphia - defied every big fix attempted.

In 2003, an influential study examined long-term trends and calculated that a difference of 100 calories a day, either ingested or spent, could tip the balance from national weight gain to weight loss. This, the researchers concluded in the journal Science, could be accomplished through small changes that the public would be more likely to embrace.

Wootan's Washington center, meanwhile, had been pondering how to get people to eat better. At a conference, she recalled, dietitians were presented with hamburgers, onion rings, and other fare from sit-down restaurants and asked to estimate caloric content. Even with nutrition degrees, they were off by hundreds of calories, always on the low side.

Wootan developed a model menu-labeling law and started calling dozens of policymakers around the country: Maine (the first to introduce a bill), New York City (the first to pass it), Philadelphia (the fourth to implement it).

City Councilwoman Blondell Reynolds Brown had been thinking along similar lines, and she was not buying the industry argument that consumers would look up carbs and fats online or ask the restaurant manager.

"I wanted it at the point of sale, because when we look at information when we order, we make different choices," Brown said. Research increasingly supports that view.

When 99 parents of children ages 3 to 6 in Seattle were randomly shown one of two hypothetical McDonald's menus - with calories listed or without - those given the additional details ordered meals for their children that contained 102 fewer calories, a 20 percent reduction, researchers reported online last week in the journal Pediatrics. There was no difference in meals the parents ordered for themselves.

Another study, from the Stanford Graduate School of Business, analyzed every transaction at every New York Starbucks in the three months before the city's menu-labeling law took effect in 2008 and in the 11 months afterward, and compared them with every transaction in Boston and Philadelphia in the same period.

It found an average reduction of 14 calories, or 6 percent, as a result of the mandate, and 26 percent among those customers who previously had tended to make high-calorie purchases. The paper, which was presented Wednesday at the University of Pennsylvania's Wharton School of Business, found no effect on profits.

Menu-labeling laws everywhere target chains, which are estimated to account for 50 to 75 percent of meals eaten out and presumably face less of a financial burden than mom-and-pop establishments. Philadelphia's ordinance covers chains with at least 15 other locations nationwide, or about 720 of the city's 5,800 restaurants.

With few menus currently listing fats, carbs, and sodium, there is little research about how consumers use the information. But public-health authorities have long sought to reduce consumption of all of them. Research published in the New England Journal of Medicine this month estimated that reducing Americans' salt intake by 3 grams (1,200 mg of sodium) a day could prevent 44,000 to 92,000 deaths a year.

Restaurants have been responding to consumer preferences, adding some reduced-calorie and gluten-free dishes. They say sodium, which acts as both a flavor enhancer and preservative, is a bigger challenge to remove from food.

And on menus, it is a challenge to add. There is just so much real estate available, said Patrick Conway, chief executive officer of the Pennsylvania Restaurant Association, who said he also worried about legal liability if dishes were slightly different from what was listed, as would be expected from different chefs.

In the 14 months since Council approved the bill, the industry has worked with the Center for Science in the Public Interest and other groups on a compromise national version that requires only calories on menus but also covers vending machines. It's in the health bills that passed the House and the Senate and that are now stalled.

Wootan is confident that national menu labeling will be approved, with or without a new health bill. If so, it will supersede all local versions.

Because federal regulations take time, the expansive Philadelphia listings would likely have a run of several years.

And then?

"Restaurants might see this as a way to draw in customers," said John Weidman, deputy executive director of the Food Trust, a local nonprofit.

Not likely, said Linda J. Lipsky, a restaurant consultant in Broomall: "Given the option, they will drop it."

Contact staff writer Don Sapatkin at 215-854-2617 or dsapatkin@phillynews.com.