The city’s health commissioner, Thomas Frieden, who was widely portrayed as the mastermind behind Mayor Bloomberg’s smoking ban, has a new target: partially hydrogenated vegetable oil.

The Department of Health and Mental Hygiene announced yesterday that it is launching a citywide campaign to get restaurants and food suppliers to stop using vegetable oils, shortening, and margarine that contain partially hydrogenated oil, because they include trans fat, which is known to increase the risk of heart disease.

The department has already sent letters to more than 20,000 restaurants and 14,000 supermarkets and food suppliers, alerting them to the dangers of trans fat and “inviting” them to make “an oil change.”

“Trans fat is artificially added to the foods that we eat and is easily removed,” Dr. Frieden said yesterday in a statement. “To help combat heart disease, the number one killer in New York City, we are asking restaurants to voluntarily make an oil change and remove artificial trans fat from their kitchens.”

A professor at the University of North Carolina’s Public Health School, Boyd Switzer, said the initiative, which also includes an education component for consumers on the science of trans fat, sounded like a “wonderful” and bold attempt to tackle a serious health risk.

He said, however, that it could prove difficult to convince bakeries that want flaky pastries, and burger joints that strive for crunchy french fries, to follow suit.

“I would imagine that the margins are tight and they’d want to use that oil as much as possible,” he said, referring to the potential price increase of an upgrade. “It’s a texture and formulation of the product.”

“When you’re talking about pie crusts and things like that, there needs to be a certain flakiness to it,” Mr. Switzer, who teaches in the school’s Nutrition Department, said. “So you’ll end up changing the character of the food somewhat, and that’s the difficulty I think chefs will have with it.”

Trans fat is are created when hydrogen is added to vegetable oil. That “hydrogenation” process is attractive for some foods because it increases the shelf life and flavor. According to the Food and Drug Administration, trans fat, like saturated fat and dietary cholesterol, raises the low-density lipoprotein – often called “the bad cholesterol.”

At the Downtown Deli on Church Street in Lower Manhattan, however, the owner, Joann Porfiris, said she couldn’t start “controlling the diets” of everybody who comes into her store.

“If they’re eating french fries, they’re eating french fries. If they’re using oil, they’re using oil,” she said as she wiped her hands on her white apron. “You’re not going to get healthy french fries.”

Ms. Porfiris, who was standing next to a 35-pound vat of soybean oil, said that the Health Department already imposes myriad regulations and that it is unrealistic to expect small businesses to keep up if they keep piling on requests, even when they are not mandatory.

“I’m sure whatever it is, it’s going to be more expensive,” she said. “Costs keep going up, and I can’t raise my prices three or four times a year. It just seems harder and harder for small businesses to stay in business.”

According to the director of the Health Department’s cardiovascular disease prevention and control program, Sonia Angell, city inspectors surveyed 529 restaurants throughout the five boroughs this spring and found that 30% used either oils or spreads with partially hydrogenated vegetable oil.

Ms. Angell said restaurants and vendors would be wise to be ahead of the curve on public health issues that are important to their customers.

Yet while the Health Department included comments from the American Heart Association and several city restaurant owners praising the initiative in a news release it sent reporters yesterday, not everyone thinks the project is a good use of public money. Department officials estimated last night that the campaign, which also included educational mailings to 100,000 residents, cost $100,000.

“It’s up to each individual to be healthy or unhealthy,” a 20-year-old Manhattanite, Sarah Rogge, who works in a downtown bakery, said.

Council Member Christine Quinn, a Manhattan Democrat who is chairwoman of the health committee, said she hoped the initiative was targeted to low-income neighborhoods, but she still wondered whether the money could have been better spent on other programs.

In the past, Dr. Frieden has not shied away from controversy. He was widely acknowledged to have lobbied the mayor to advance the smoking ban, which many believed was an inappropriate government intervention, and one that hurt some bars and restaurants. And he has regularly clashed with the council over budget priorities.

During his tenure, the department has launched an aggressive public health awareness campaign, which, among other things, focuses on quitting smoking and eating in a healthful way. Dr. Frieden’s letter to vendors instructs those who want more information to call 311 and “ask for trans fat.”

The city campaign piggybacks on two recent federal initiatives. New government dietary guidelines recommend that trans fat be kept to a minimum. Starting next year, the Food and Drug Administration will begin requiring food manufacturers to list the trans fat content on all products. In 2003, roughly 23,000 city residents died of heart disease.