Trans fats, or trans fatty acids are formed mainly by industrial processes that convert vegetable oils into a more solid fat. The food industry loves them because they have a long shelf life, stay solid at room temperature so that your package of cookies does not become an oily mess, and are more desirable when used for deep frying.
Doctors have been warning about the health dangers of trans fats since conclusive studies were published in 2006. Laws were passed requiring labeling of products containing trans fats to allow consumers to avoid these disease causing fats, but there were loop-holes in the labeling laws. A new study just published (link) by the Centers for Disease Control and Prevention (CDC) finds that because of these loopholes, that almost 10% of the thousands of packaged processed foods that they looked at contained trans fats, even though the label said "0 grams" of trans fat per serving.
The other gap in the labeling rules involves the trans fat reported in packaged fried foods like chips. Trans fats are produced by heating vegetable oils in the presence of oxygen. Even though the fats used to fry chips may not have started out as trans fats, trans fats are generated during the process of frying, and these trans fats never show up in the labeling. (This study from the CDC does not discuss this continued source of trans fats in processed fried foods.)
The only thing you can do for now is avoid packaged fried foods, and read labels diligently. If the label reports "partially hydrogenated" fats as an ingredient, then the product contains trans fats, even if the nutrition label says "0 grams" of trans fats. Stay away from these foods.
