Partially hydrogenated oils are cheaper than saturated animal fats like butter, and for years were thought to be healthier. They are formed when liquid oil is treated with hydrogen gas and made solid. They became popular in fried and baked goods and in margarine. Crisco, originally marketed in the beginning of the 20th century, was the archetype, although it now contains no trans fat.

But over the years, scientific evidence has shown they are dangerous because they raise the levels of so-called bad cholesterol and can lower the levels of good cholesterol. In 2003, the F.D.A. required that artificial trans fats be listed on food labels, a shift that prompted many large producers to eliminate them. Two years later, New York City under Mayor Michael R. Bloomberg told restaurants to stop using artificial trans fats in cooking; other places, including California, Cleveland and Philadelphia, followed suit. Many major chains, like McDonald’s, found substitutes and sharply reduced the use of trans fats.

Those actions led to a stunning reduction in consumption: Americans ate about one gram a day last year, down from 4.6 grams in 2006. A report from the Centers for Disease Control and Prevention found that blood levels of trans fatty acids among white adults in the United States declined by 58 percent from 2000 to 2009.

But the fats were not banned. They are required to be listed on food labels only if there is more than half a gram per serving, a trace amount that can add up. Even as little as two or three grams of trans fat a day can increase the health risk, scientists say. (Some trans fats occur naturally. The F.D.A. proposal applies only to those that are added to foods.)

“The artery is still half clogged,” said Dr. Thomas R. Frieden, the director of the disease centers, who led the charge against the fats in New York when he was health commissioner there.

Scientists emphasized that saturated fats are still an enormous problem in the American diet, and that Thursday’s ruling should not give consumers false security.

“In the push to reduce trans fats, people have been forgetting that saturated fats are much worse because there is a lot more saturated fat in the diet than trans fat,” said Dr. Scott Grundy, director of the Center for Human Nutrition at the University of Texas Southwestern Medical Center in Dallas.