First New York City required restaurants to cut out trans fat. Then it made restaurant chains post calorie counts on their menus. Now it wants to protect people from another health scourge: salt.

On Monday, the Bloomberg administration plans to unveil a broad new health initiative aimed at encouraging food manufacturers and restaurant chains across the country to curtail the amount of salt in their products.

The plan, for which the city claims support from health agencies in other cities and states, sets a goal of reducing the amount of salt in packaged and restaurant food by 25 percent over the next five years.

Public health experts say that would reduce the incidence of high blood pressure and should help prevent some of the strokes and heart attacks associated with that condition. The plan is voluntary for food companies and involves no legislation. It allows companies to cut salt gradually over five years so the change is not so noticeable to consumers.

“We all consume way too much salt, and most of the salt we consume is in the food when we buy it,” said Dr. Thomas Farley, the city health commissioner, whose department is leading the effort. Eighty percent of the salt in Americans’ diets comes from packaged or restaurant food. Dr. Farley said reducing salt from those sources would save lives.

Since taking office, Mayor Michael R. Bloomberg, who just began his third term, has gained a reputation as an advocate for healthy living, initiating prominent campaigns against smoking and harmful trans fats. To combat obesity, he has campaigned for calorie labeling on restaurant menus and warned consumers about sugary soft drinks.

The city’s salt campaign is in some ways more ambitious and less certain of success than the ones it waged against smoking and obesity. For one thing, the changes it prescribes require cooperation on a national scale, city officials said, because major food companies cannot be expected to alter their products for just the New York market.

And removing salt from many products can be complicated. Salt plays many roles in food, enhancing flavor, preventing spoilage and improving shelf life. It helps bread to rise and brown.

The city’s campaign against salt resembles its push to cut trans fat from restaurant foods, which began with a call for voluntary compliance. When that did not work, the city passed a law to force restaurants to eliminate trans fat.

But city officials said it would be difficult to legislate sodium reduction.

“There’s not an easy regulatory fix,” said Geoffrey Cowley, an associate health commissioner. “You would have to micromanage so many targets for so many different products.”

He said officials hoped the campaign would work through public pressure. Companies that complied would benefit from good publicity.

The city has been discussing the program with the food industry since late 2008, yet only a few companies appear ready to jump on board. One of those is A.& P., the supermarket chain.

“We think it’s a very realistic set of criteria that our suppliers can adhere to,” said Douglas A. Palmer, vice president for store brands at A.& P.

He said the company expected to embrace the city’s salt reduction goals for the hundreds of store brand products it sells under labels like America’s Choice and Smart Price in 435 supermarkets throughout the Northeast and Mid-Atlantic regions. In Manhattan, the chain operates under the name Food Emporium.

Subway, the fast food sandwich chain, also said it expected to commit to the city’s salt guidelines at its nearly 23,000 stores across the country.

Lanette R. Kovachi, Subway’s corporate dietitian, said the company has reduced salt in stores in several other countries, including Britain and Australia, in response to government programs there.

“We view these as achievable goals,” she said.

The company’s best-selling item, a six-inch turkey sandwich, is already below the city’s five-year average target for lunch meat sandwiches in restaurants. But the chain also has a six-inch spicy Italian sub whose salt content is well above the city’s goals.

On Monday, after a year of consultations with industry, the city will release preliminary targets for sodium content. After a review, the city will unveil final targets in the spring and ask companies to commit to the program.

The system proposed by the city is complex, with reductions ranging from 10 to 40 percent for 61 classes of packaged foods and 25 classes of restaurant foods.

It would measure the average salt content of a company’s entire line of a particular type of product, like canned vegetables, breakfast cereals or frozen dinners, adjusted to give greater weight to products with the highest sales. That would allow companies to maintain a range of sodium levels but would create incentive to cut back on salt in the most popular items.

While most food companies say they agree at least with the goal of reducing salt, some medical researchers have questioned the scientific basis for the initiative, saying insufficient research had been done on possible effects. While agreeing that reducing salt is likely to lower average blood pressure, they say it can lead to other physiological changes, some of which may be associated with heart problems.

An elaborate clinical trial could weigh the pluses and minuses of cutting salt in a large group of people. But that would cost millions, and it has not been done.

Dr. Michael H. Alderman, a professor at the Albert Einstein College of Medicine, said the city’s initiative, if successful in reducing salt, would amount to an uncontrolled experiment with the public’s health.

“I’m always worried about unintended consequences,” he said.

The federal government recommends that sodium intake from salt be limited to 1,500 to 2,300 milligrams a day, with the latter figure equaling about a teaspoon. But the average adult in this country consumes about 3,400 milligrams a day.

Several major companies, including some that have been leaders in reducing salt, said they would not join the city initiative.

“One of the things we want to bring across to New York City is that sodium reduction does not always follow a prescribed time or prescribed progress,” said Chor-San Khoo, vice president for global nutrition and health at the Campbell Soup Company. “There’s no one size fits all.”

Campbell has already made significant reductions in the amount of salt in many of its products, including many canned soups, V8 beverages and Pepperidge Farm breads.

“We will continue to reduce sodium as long as there’s consumer acceptance in the marketplace,” Ms. Khoo said.

ConAgra, which makes a wide array of products, including Hunt’s canned tomato products and Chef Boyardee packaged meals, said it would continue with previously announced plans to cut the sodium in its portfolio of products by 20 percent by 2015.

“We don’t have plans to join other organizations’ pledges,” the company said.

Pass On the Salt