The FDA, which will collect public comment for 60 days, proposes to remove trans fats, also called partially hydrogenated oils, from the list of substances "generally recognized as safe" in food.
"That changes the burden of proof," said Dariush Mozaffarian, a cardiovascular epidemiologist at Harvard University. "If a company wants to use trans fats, they'll have to prove they are not harmful."
University of Pennsylvania cardiologist Mariell Jessup, speaking as president of the American Heart Association, said the group has long advocated getting rid of trans fats.
"I believe that the cumulative scientific data against trans fats is just so overwhelming, it just called for action by the FDA," Jessup said.
Trans fats are formed when hydrogen is added to vegetable oil to make it solidify. The hydrogenation process makes fats less likely to spoil, and makes foods feel less greasy.
Trans fats are common in processed foods such as crackers, frozen pizzas, margarines, and coffee creamers, as well as fried foods like french fries and doughnuts. Food manufacturers use hydrogenated oils because they're cheaper than alternatives such as butter, and give products a longer shelf life.
But these man-made fats also raise blood levels of low-density lipoprotein, the so-called bad cholesterol, while lowering good cholesterol.
"They activate liver production of fats," Mozaffarian said. "When you eat any other type of fat, your liver production goes down cause you're eating fat. So trans fats cause fatty liver," which is linked to obesity and liver disease.
As evidence of harm has mounted over the last 20 years, public health advocates such as the Center for Science in the Public Interest have sued fast-food chains, campaigned for local and state prohibitions on trans fat, and pushed to list it on product nutrition labeling.
Slowly and in piecemeal fashion, the push succeeded. In 2006, for example, the FDA required that trans fats be listed on food labels, although products with less than 0.5 gram per serving are exempted. That same year, New York City told restaurants to stop using it in cooking.
Philadelphia followed suit in 2007, requiring city restaurants and commercial kitchens to give up trans fats. (When mom-and-pop bakeries complained the ban would ruin their heirloom recipes, City Council granted them an exemption.)
If trans fats are forbidden, "I guess we'd have to find a solution," said James Hausman, owner of Philadelphia-based Swiss Haus Bakery, which is known for its hazelnut sponge cake. "Manufacturers [of shortening] would end up reformulating and we'd be forced to change our recipes."
By this year, a Harvard study led by Mozaffarian found dramatic changes in the processed food supply. Of 270 brand name products, two-thirds were reformulated between 2007 and 2011 to reduce or eliminate the bad fats. Tastykake was among companies that made the most progress.
As a result, Americans' trans fat intake fell from almost five grams a day in 2006 to one gram a day last year, according to the Centers for Disease Control and Prevention.
The FDA's proposed action would not completely eliminate trans fat from the American diet because it occurs naturally in some meats and dairy products.
But declaring the artificial version unsafe would end a long debate.
"Not only is artificial trans fat not safe, it's not remotely necessary," said Michael Jacobson, executive director of the Center for Science in the Public Interest.