Produced when liquid oils are processed into solid shortenings, trans fats (also known as partially hydrogenated oils) raise LDL, or "bad," cholesterol and lower HDL, or "good," cholesterol.
Credit: Randy Mayor; Leigh Ann Ross

Since January 2006, the Food and Drug Administration has required all food manufacturers to indicate the amount of trans fat in a serving of food. (Food with less than one-half gram of trans fat per serving can be labeled "trans fat-free.")

Sources: Foods can harbor trans fats if they're made with partially hydrogenated oils. Some meat and dairy products contain trace amounts of naturally occurring trans fats. It's unknown whether these fats have the same harmful effects to your health as manufactured trans fats.

What you need to know: Harvard's landmark 2005 Nurses' Health study, which followed nearly 79,000 women, was one of the first to find that trans fats raise LDL cholesterol, lower HDL cholesterol, and increase heart disease risk. Now a 2007 Harvard report confirms that the more trans fats you consume, the greater your risk. Women with the highest levels of trans fat in their red blood cells (from eating a diet containing high levels of trans fats) have three times the risk of developing heart disease as women eating the least trans fats. Harvard cardiologist Dariush Mozaffarian, MD, DrPH, thinks the trans fat picture isn't yet complete. Preliminary reports show these artificial fats are likely to raise insulin resistance (a precursor to diabetes), increase fat around the abdomen (the worst type of fat for health), and promote inflammation.

Cooking strategies: Since even small amounts of trans fats are harmful to your health, Mozaffarian thinks it's best to avoid eating foods made with partially hydrogenated oil. Focus instead on whole foods―fresh produce, fish, liquid vegetable oils, and whole grains―all of which are naturally trans fat-free.