The New York City Board of Health voted unanimously yesterday to move forward with plans to prohibit the city’s 20,000 restaurants from serving food that contains more than a minute amount of artificial trans fats, the chemically modified ingredients considered by doctors and nutritionists to increase the risk of heart disease.

The board, which is authorized to adopt the plan without the consent of any other agency, did not take that step yesterday, but it set in motion a period for written public comments, leading up a public hearing on Oct. 30 and a final vote in December.

Yesterday’s initiative appeared to ensure that the city would eventually take some formal action against artificial trans fats. If approved, the proposal voted on yesterday by the Board of Health would make New York the first large city in the country to strictly limit such fats in restaurants. Chicago is considering a similar prohibition affecting restaurants with less than $20 million in annual sales.

The New York prohibition would affect the city’s entire restaurant industry, by far the nation’s largest, from McDonald’s to fashionable bistros to street corner takeouts across the five boroughs.

The city would set a limit of a half-gram of artificial trans fats per serving of any menu item, sharply reducing most customers’ intake. The fats are commonly found in baked goods, like doughnuts and cakes, as well as breads and salad dressing.

Officials said that the typical American diet now contains 5.8 grams of trans fats per day, and that a single five-ounce serving of French fries at many restaurants contained 8 grams of trans fats.

Members of the Board of Health, all mayoral appointees, expressed vigorous support for the proposal, which was drafted by the Department of Health and Mental Hygiene. The board members said that the initiative could set an example for the nation, and that New York City should play a leading role because of its high rate of heart disease and because New Yorkers consume more restaurant meals and takeout food than most Americans.

The proposal met immediate resistance among restaurant owners, who said banning trans fats would raise their costs and change the taste of some items. “I’m wondering if there are grounds for a lawsuit,” said E. Charles Hunt, executive vice president of the New York State Restaurant Association, which represents about 3,500 restaurants in the city.

The Board of Health vote comes a year after it conducted an unsuccessful campaign to persuade restaurants to eliminate trans fats from their recipes voluntarily. It said yesterday that despite mass mailings about the hazards of trans fats and training programs for 7,800 restaurant operators, about half the city’s restaurants continued to serve trans fats, about the same as before the campaign.

Trans fats, derived from partially hydrogenated oils, became popular in the 1950’s as an alternative to the saturated fats in butter. They allow fast-food restaurants to use frying oil for longer periods and make crunchier cookies and flakier pie crust. They also have a longer shelf life than butter, olive oil, corn oil or other alternatives.

Health officials said yesterday that many healthier alternative cooking ingredients had been developed that would cost little more than trans fats, and have little or no effect on taste.

Lynne D. Richardson, a member of the Board of Health and a professor of emergency medicine at Mount Sinai School of Medicine, said yesterday that restaurant owners might still see an advantage in the long shelf life of trans fat products.

“But human life is much more important than shelf life,” she said. “I would expect to see fewer people showing up in the emergency room with heart attacks if this policy is enacted.”

If the measure is adopted in December, health officials said, the restrictions would be phased in. Restaurants would be given until July to eliminate oils, margarines and shortening from the recipes that contain more than a half-gram of trans fat per serving.

They would also have until July 2008 to remove all menu items that exceed the new limit, including bread, cakes, chips and salad dressing. The only exclusions from the restrictions would be packaged food items, like candy, that remain in the manufacturers’ original packaging when served, as well as naturally occurring trans fats, which are found in some meats and dairy products.

Health officials said that the regulations would be enforced by restaurant inspectors, who would examine kitchens for products with trans fat, but that there would be no attempt to test prepared foods.

The New York City proposal comes at a time when companies in the packaged food industry, under pressure from health advocates, have reduced the use of trans fats. A recent ruling by the federal Food and Drug Administration requires all food companies to include trans fat levels in labeling information.

Several restaurant chains, including Wendy’s, Starbucks and Subway, have announced efforts to eliminate or sharply reduce trans fats. McDonald’s, which has not, “will closely examine the board’s proposal,” said Walt Riker, a company spokesman.

“Concerning trans fats, McDonald’s knows this is an important issue, which is why we will continue to test in earnest to find ways to further reduce” their use, he said.

For the Board of Health, the trans fat plan is the latest in a series of regulations that have placed New York City in the forefront of regulating behavior and products’ content in order to benefit public health.

Three years ago, the city banned smoking in restaurants, a measure angrily protested by some restaurant owners, but it led to similar bans in several other cities. Yesterday, health officials compared the restrictions on trans fats to the city’s 1960 prohibition on the use of lead paint, years before it was banned in most of the country.

“Like lead paint, artificial trans fat in food is invisible and dangerous, and it can be replaced,” said Thomas R. Frieden, the city’s health commissioner, after the Board of Health vote yesterday. “No one will miss it when it is gone.”