It is probably the most frequent question that any nutritionist is asked. Although they both are sources of fat, to determine which is better for your health you have to look more closely at what kind of fats are found in butter and margarines.
Vegetable oils are the base for margarines.
Soybean, olive, corn and canola are common oils used in the manufacture of margarines. But oils are liquid at room temperature. In order to produce solid margarine, unsaturated fats in the oil are converted to saturated fats by a process called hydrogenation. The level of unsaturated fats goes down in margarines and the level of saturated fats goes up. Even more important is the fact that during the process of hydrogenation to produce margarines, trans fatty acids are produced.