To be honest, we don’t yet know for sure. If you’re consuming them, you’re part of a 40-year social experiment to find out.
The following material is derived from Dr. Robert Erdmann’s book, Fats that Can Save Your Life (BioScience, Port Orchard, WA, 1990). It is relevant to anyone who consumes fat as part of their diet. The section on refining is pp 66-69.
In a word: metabolic poisons. Consuming partially hydrogenated oils is like inhaling cigarette smoke. They will kill you — slowly, over time, but as surely as you breathe. And in the meantime, they will make you fat!