NEW YORK - The era of the supersized cola may come to an end in New York City on Thursday, when health officials are expected to approve an unprecedented 16-ounce (470-milliliter) limit on sodas and other sugary drinks at restaurants, delis and movie theaters. But will it actually translate into better health?

Doctors and nutrition experts said the regulation's success or failure may depend on more than just the modest number of calories it might slash from people's diets. It will hinge on whether the first-in-the-nation rule starts a conversation that changes attitudes toward overeating.

"Ultimately it does come down to culture, and it comes down to taking some first steps," said Dr. Jeffrey Mechanick, a professor at the Mount Sinai School of Medicine who has studied the effect of government regulation on the obesity epidemic.

"There are so many factors that are acting in this complex disease. Obesity is not just a disease simply of people drinking too much sugary soft drink," he said. "Just attacking one thing, individually, isn't going to do much."

But if the rule is part of a broader social and scientific assault on the dangers of too much sugar, he said, it could be tremendously effective. He likened it to the drumbeat about the dangers of smoking, which took decades to translate into results.

"People talk about it. It gets ruminated at social parties. It gets ruminated in politics and the media. And all of a sudden, you have an awareness," he said.

City health officials say that by restricting portion sizes for sugary beverages, they are taking on one of the leading culprits in the national fat problem.

Since the mid-1970s, Americans have increased their daily intake by 200 to 300 calories while getting less exercise -- a couch-potato lifestyle that has left the country with epidemic levels of obesity and diabetes.

While plenty of foods contribute to the problem, some experts believe soft drinks deserve a greater share of the blame, in part because the body doesn't scream, "I'm full!" when someone downs a 32-ounce soda, even though it has more calories than a typical fast-food cheeseburger.

The standard soda has gone from a 12-ounce (354-milliter) can in the 1980s to a 20-ounce (591-milliliter) bottle today.

"This is the largest single driver of the obesity epidemic," said New York City Health Commissioner Dr. Thomas Farley said. "It is the largest source of added sugars to our diet."

The math behind the ban is simple:

A 16-ounce (470-milliliter) Coke has 200 calories. A 20-ounce (591-milliliter) Coke has 240 calories, or about 30 more than a Hershey bar.

If you drink a soda per day - as do 46 percent of Bronx residents, according to one recent Health Department survey - choosing the 16-ounce (470-milliliter) bottle rather than the 20-ounce (590-milliliter) would save you 14,600 calories a year, or the equivalent of 70 Hershey bars. That is enough to add about four pounds (1.8 kilograms) of fat to a person's body.

To burn off those extra calories, an average-size woman would have to walk about 340 miles (550 kilometers).

New York City health officials aren't actually expecting an effect that large, in part because the ban doesn't apply to sodas sold in grocery stores.

But if the average New Yorker decreased soda consumption from 20 ounces (591-milliliter) to 16 ounces (470-milliliter) only once every two weeks, that would still be enough to prevent roughly 2.3 million pounds (1 million kilograms) of weight gain in the city every year, Farley said.

The rule, which would take effect next March, would have its biggest effect on fast-food joints like McDonald's, where a 16-ounce (470-milliliter) drink is considered a small.

The new rules would be enforced through the city's existing system for inspecting restaurants. Restaurants with self-serve soda fountains will be restricted to giving out 16-ounce (470-milliliter) cups, but free refills will still be allowed.

McDonald's would not discuss the possible effect on soda sales or how it might reconfigure cup sizes. It issued a statement expressing displeasure with the proposed rule but declined to say whether it would fight it.

Some advocates for the beverage and restaurant industries have said they are contemplating suing to halt the ban.

"Public health issues cannot be effectively addressed through a narrowly focused ban," McDonald's said in its statement.

Many doctors would agree.

The rule, for example, doesn't apply to alcohol, which some studies have indicated plays a larger role in obesity among adults. Nor does it address the thousands of other things we eat that are terrible for us, such as potato chips or fries.

"Beverages are the low-hanging fruit," acknowledged Barry Popkin, a professor of nutrition at the University of North Carolina and an author of books on global obesity. "I don't believe that cutting the portions alone will do it all."

But he said the real importance of the rule is that it has started a national dialogue.

"The bigger effect is helping to continue to educate Americans about why we have to do this," he said. "There is a chance this is going to matter."