What are trans fats, and why are they unhealthy?

WASHINGTON -- You may not even know you are eating them, but trans fats will soon be mostly gone from your food. The Food and Drug Administration said it will require food companies to phase them out over the next three years because the agency says they are a threat to public health.

Among the foods that commonly contain trans fats: frostings, pie crusts, biscuits, microwave popcorn, coffee creamers, frozen pizza, refrigerated dough, vegetable shortenings and stick margarines. The fats help give a more solid texture and richness to certain foods, such as baked goods and ready-to-eat frostings.

Questions and answers about the dangerous fats:

What are trans fats?

Trans fats are created when hydrogen is added to vegetable oil to make it more solid, which is why they are also called partially hydrogenated oils. They can contribute to heart disease and are considered even less healthy than saturated fats, which also can contribute to heart problems.

Why are they so bad?

Trans fats can raise "bad" cholesterol and lower "good" cholesterol. That can contribute to heart disease -- the leading cause of death in the United States.

How will they be phased out?

The FDA has determined trans fats no longer fall in the agency's "generally recognized as safe" category, which is reserved for thousands of additives manufacturers can put in foods without FDA review. Once trans fats are off the list, anyone who wants to use them would have to petition the agency for a regulation allowing it.

Will they be completely banned?

No. Food companies can petition the FDA to use them. The Grocery Manufacturers Association, the main trade group for the food industry, is working with companies on a petition that formally would ask the FDA whether it can say there is a "reasonable certainty of no harm" from some specific uses of the fats. But the agency isn't likely to approve many uses, since it has determined the fats are a threat to public health.

Some trans fats in the food supply occur naturally in meat and dairy products. The FDA has not targeted those small amounts because they would be too difficult to remove and aren't considered a major public health threat by themselves.

Haven't most of them been phased out already?

Yes. The FDA says between 2003 and 2012, people ate about 78 percent less trans fat as food companies began using other kinds of oils to replace them.

Why is the FDA doing this?

The FDA is aiming to get rid of those trans fats left in the marketplace, saying they are still a public health concern. While the fats have been phased out in many foods, some companies still use them.

How do I know I am eating them?

The FDA has required the amount of trans fats in foods to be listed on the backs of food packages since 2006, but that doesn't always tell the whole story. Companies are allowed to round less than half of a gram of trans fat to zero on the package label. Susan Mayne, director of the FDA's Center for Food Safety and Applied Nutrition, says those hidden amounts still can "add up to a considerable intake of trans fats if you look at the overall diet."

For now, the agency is recommending consumers take a look at ingredient lists on packaged foods to make sure they don't contain partially hydrogenated oils. Once the three-year compliance period is up, none of those ingredients would be allowed unless FDA specifically approves them.

Are all fats bad for you?

No, but they should be eaten in moderation. Unsaturated fats found in nuts, vegetable oils and fish are the best for you. Saturated fats mostly derived from animals are less healthy and should be less than 10 percent of a person's daily calories.

Is it hard to find substitutes?

In some cases, no. Frying oils are substituted easily, and food scientists have figured out how to substitute other fats for trans fats in many items. In other cases, it will be harder. Ready-to-eat cake frosting, for example, gets some of its solid shape from trans fats.

Will I notice the change?

Probably not. Trans fats don't have any particular taste, and in most cases other fats will be substituted. Your heart might notice, though. The Obama administration says the move will reduce coronary heart disease and prevent thousands of fatal heart attacks every year.