This is the problem with margarine
"Hydrogenated fats are created when an oil that is largely unsaturated, such as corn oil, has hydrogen added to it, causing fat to become more solid at room temperature.
During hydrogenation, the unsaturated fat becomes more saturated.
"In addition, processed foods made with hydrogenated oils pose another health hazard: trans fatty acids."
Dr. Leonard Lopez writes, "The really bad saturated fat is called a trans fatty acid. These are chemically altered (processed) fats. You can find them in most packaged foods listed on the label as partially hydrogenated or hydrogenated oil. Our bodies have a more difficult time with processed foods and would prefer foods in their natural state."
The more solid and hydrogenated the fat, the more trans fatty acids there are in the product.
What do trans fatty acids do to us?
Among other results, researchers have found that trans fatty acids significantly raise LDL cholesterol levels, the bad cholesterol, while lowering the HDL levels, the good cholesterol. In the Framingham Heart Study (a 40 year study covering 5,209 individuals living in Massachusetts) high LDL cholesterol levels combined with low HDL levels was indicative of coronary heart disease risk. "
That was taken from a bit of a cranky site but the science is sound. There have been lots of studies to show that hydrogenated fats are a bad idea.