Vegetable shortenings are a blend of many types of vegetable oils. The oil becomes solid or firm through the process of hydrogenation. The result is a product that has a lower polyunsaturated fatty acid content and a higher saturated fatty acid content than liquid vegetable oils. Hydrogenation also forms monounsaturated fats that have a different physical arrangement. These trans monounsaturated fatty acids are similar in stability to saturated fatty acids.
Research shows their effect on blood cholesterol levels is variable.
From ten percent to almost one-third of margarines and hydrogenated vegetable shortenings are trans fatty acids. Vegetable shortenings are less desirable than vegetable oils as a food fat because of their fatty acid makeup. Similarly, margarines are also blends of liquid vegetable oils and partially hydrogenated vegetable oils. To keep saturated fatty acid content to a minimum use margarine that lists liquid vegetable oil first in its ingredient label. In general softer margarines are a better choice than those which are harder.