Think twice before you reach for those fat-free cookies. Many foods that are listed as having zero trans fat still contain the unhealthy ingredient. A new study found that approximately one in 10 processed foods sold in the U.S. still contain trans fat.

The new study, released by the Centers for Disease Control and Prevention (CDC), found that consumers are unknowingly still eating trans fats even if they aren't listed on the label.

Researchers analyzed nutritional information of 4,340 popular packaged foods in the U.S. The packaged processed foods that were analyzed included cookies, frozen entrees, frozen pizza, salad dressing, margarine and canned soups.

They discovered that nine percent of the foods listed hydrogenated oils, the main source of trans fat, in its list of ingredients. Trans fat increases "bad" cholesterol levels, which can clog arteries and decreases the "good" cholesterol levels. "Government efforts to eliminate partially hydrogenated oils from packaged foods will substantially reduce exposure to this known cardiovascular disease risk factor," the CDC writes.

Labeling laws allow products to claim to have zero trans fat if they have less than 0.6 grams per serving. The study found that 84 percent of the products analyzed used this loophole to claim zero trans fat on their labels.

"Industrial trans fat is still common in U.S. packaged foods" and "most products that contain PHOs are labeled as containing 0 g of trans fat," the researchers write.

Consumers are encouraged to watch out for the words "partially hydrogenated" or "PHO" on product labels that claim to have no trans fats.

The Food and Drug Administration announced that partially hydrogenated oils are not "generally recognized as safe" for the time being. It is currently in the process of determining if packaged foods and restaurant food will be allowed to contain partially hydrogenated oils.

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