Food labels quite commonly list "hydrogenated oils" (or fats), and even more often "partly hydrogenated oils" among the ingredients. Here's in little more details what they are, and how do they affect your health.
The term "hydrogenation" comes from unsaturated fats being transformed into saturated by adding more hydrogen atoms to their molecular structure. This makes oil stable, and unhealthy; it destroys most nutrients, while creating some trans-fatty acids and other altered (unhealthy) fat molecules.
Fats are often "partially hydrogenated", which means that the process of hydrogenation is deliberately stopped at some point before completion, in order to achieve certain level of oil/fat consistency. This is how margarines, shortenings and partially hydrogenated vegetable oils (most of commercial sunflower, safflower, and corn oil) are made.
All of these oils are high in Omega-6 essential fatty acids (linoleic acid) when in their natural form; if you don't see it on the label, pass it, even if it doesn't say that the oil is hydrogenated. It is just to much of a valuable selling point to be omitted, and indicates that oil has been processed in order to destroy unstable Omega-6 fatty acids.
And, don't think - as I used to - that "partially hydrogenated" is less bad than "hydrogenated". It is exactly the opposite. While most of the fully hydrogenated fat is in a relatively harmless saturated form, partially hydrogenated fats have significantly higher proportion of toxic, twisted fatty acids in transition from unsaturated to saturated form (hence "trans-fatty").
The worst of commercial bad fats is probably margarine. It can contain in excess of 60% trans-fatty acids, which doesn't prevent many doctors from recommending it to their patients suffering from cardiovascular disease.
While the excuse for it is that margarine does not contain cholesterol, this long-established medical practice (in the US, not in Europe anymore) is, at the present level of scientific knowledge, inexcusable. Trans-fatty acids do increase total cholesterol and decrease HDL ("good") cholesterol, while interfering with liver function and joining free radicals in causing damage at the cellular level.
Another health risk of
hydrogenated fats, either fully or partially, is that they are often contaminated with metal
catalysts used for hydrogenation, nickel and aluminum.