Margarine contains significant amounts of trans fatty acids. These hydrogenated fats have a different form than saturated fats but act like them when it comes to clogging arteries. They pose an equal or greater heart-disease risk to consumers than saturated fats.
Although margarines are generally made with vegetable oils that are high in unsaturated fats and low in saturated fats, food manufacturers hydrogenate many of the mono-unsaturated fats in order to solidify a liquid oil into a spread. Margarine makers also know that a hydrogenated oil has a longer shelf life because it is less susceptible to oxidation and hence to rancidity.
Hydrogenation changes the form of a mono-unsaturated fat by adding hydrogen atoms to its hydrocarbon molecular chain. This converts the unsaturated fat molecule into a trans fatty acid molecule.
Therefore, do not be hoodwinked by a company's sales pitch that its product is made totally with unsaturated oils. What counts is not the absence of saturated fatty acids in the oil when it enters the margarine factory, but the presence of trans fatty acids in the product when you melt it over your steaming corn on the cob. If you see the word "hydrogenated" or the phrase "partially hydrogenated" preceding "vegetable oils" in the ingredient list, you know the product contains trans fatty acids. On the other hand, if you do not see the "hydrogenated" qualifier, do not automatically assume that the food has not been hydrogenated.
Generally, the harder the margarine, the more trans fatty acids it contains. Therefore, other factors being equal, pourable margarine has less trans fatty acids than tub margarine, which in turn has less than stick margarine.