A city proposal to ban New York restaurants from selling food containing an unhealthy artificial fat could open a new front in a national fight over the safety of America's food supply, legal experts say.

In recent years, states and a few cities interested in ridding kitchens of suspected toxins have become increasingly bold about mandating warning labels for potential hazards such as lead in candy, mercury in fish and pesticides in vegetables.

Some of those measures have prompted fierce opposition from the food industry and members of Congress who say the states are exceeding their authority.

Experts said New York City would take the boldest step yet if its Board of Health approves a proposal to ban restaurants from preparing foods containing more than trace amounts of artificial trans fatty acids.

Under a plan announced last week, chefs at thousands of restaurants would be barred from using partially hydrogenated oil, which has been found in some shortenings and frying oils for decades.

Artificial trans fatty acids are thought to cause cholesterol problems and some studies have blamed them for an epidemic of heart disease deaths.

But federal regulation has been light, and public health law experts said they were stunned that New York would ban a substance the Food and Drug Administration only began listing on food labels this year.

Lawrence O. Gostin, an associate dean at Georgetown University's law school and director of the Center for Law and the Public's Health, called the proposal "breathtaking."

He said it was sure to prompt a lawsuit challenging the city's authority to enact such a measure. Big fast food companies that use artificial trans fats to prepare French fries, muffins and doughnuts might also sue over the potential impact of the rules on interstate commerce, he said.

"Certainly if there is a local deli in New York that is regulated by the local health department, it is clearly for the city to decide what is safe and what isn't," Gostin said, "But if you're talking about large chains like McDonald's or Burger King … then there are powerful questions of federalism at stake.

"On the other hand," he added, "when the federal government refuses to act or neglects to act in the face of a major health crisis, then sometimes you need cities and states to step in to the vacuum and protect the public."

Authority not clear

Anthony M. DiLeo, a professor of health-care law at Tulane Law School who also teaches at Tulane Medical School, said public health agencies have a well-established right to ban items that are inherently dangerous, like spoiled food or lead in paint.

But the limits of a city's authority when it comes to something like trans fat are less clear, he said.

"You get to something here that is not a bacteria, it is not a virus, it is not an immediate danger … One meal containing a trans fat is not dangerous, per se," DiLeo said.

"If you have the authority to ban that, you would have to assume you have the authority to ban all sorts of things that, in small amounts, can't be harmful, but in large amounts could be."

City Health Commissioner Thomas Frieden said he is confident the ban could survive any legal challenge.

The U.S. Supreme Court has held that health departments have the authority to prohibit the sale of foods that are impure, unfit for use or spread disease.

Artificial trans fats, invented as a substitute for natural animal fats like butter or lard, have more in common with cancer-causing agents than with other foodstuffs that can be unhealthy if consumed in gluttonous amounts, like saturated fats or salt, Frieden said.

"If these were cancer deaths, people would react very differently," he said.

Members of the public may weigh in on the proposed ban over the next few months. It is not expected to come before the Board of Health for a vote until at least December.

Yet to be seen is whether the proposal will attract the attention of Congress, which has frowned lately on attempts by the states to aggressively regulate food safety.

One bill, passed in the House and now under consideration in the Senate, would prevent states from requiring food labels tougher than those already approved by the federal government.

Under Canada's food labelling system, pre-packaged foods and drinks must list information on calories, fat, saturated and trans fats, cholesterol, sodium, carbohydrates, fibre, sugar, protein, calcium, iron, vitamin A and C in a nutrition box.

However, food producers such as restaurants and bakeries do not have to disclose any nutritional information.