A friend of mine, who is a retired microbiologist, explained something about trans fats to me. I’m going to assume he knows whereof he speaks.
Vegetable fats are (with a few exceptions) UNsaturated. They are liquid at room temperature. Animals make saturated fats, which tend to be solid at room temperature. About a hundred years ago, Proctor & Gamble introduced Crisco, hydrogenated vegetable oil, revolutionizing the culinary experience and quite possibly poisoning a generation or more of American consumers.
Hydrogenation turns unsaturated fat into saturated fat. BUT there’s a nasty fact: Each molecule of saturated fat thus created is randomly cis or trans. Trans fat is sticky. It sticks to itself. It sticks to anything it touches. It sticks to your arteries. It is evil. It does not occur (much) in nature, which leads to a nasty result (discussed below).
Animals make saturated fats too. But by a very different process: Instead of subjecting veg oil to hydrogen under great pressure (the artificial way), animals use enzymes to do their chemistry. This is a MUCH more specialized and selective process, and does NOT create trans fats (other than the occasional stray molecule).
Now, what happens when you get yourself full of trans fat that you’ve eaten: Other kinds of fat, you can metabolize. This is done with enzymes too. But enzyme chemistry is a very specialized and selective process, remember? Your enzymes cannot metabolize trans fat. Since they don’t occur in nature, nobody ever evolved the mechanisms to do it. So the trans fat stays in your system since your body doesn’t know how to do anything with it. And it sticks to, and gums up, everything it touches. So it accumulates in your body over the years. If you quit eating it, I guess your body very gradually can eliminate some of it (e.g., by pissing it away) over time.
The conclusion I drew from hearing about that was, that trans fat is a portion of artificially saturated fat, and is probably the main reason that saturated fat was ever so dangerous in the first place. I conclude that merely saturated (non-trans) fat by itself, as present in animal fat, probably isn’t so bad. It’s the trans that kills.
ETA: You can easily google up many articles discussing this. Without having read them myself, I won’t stick my neck out and post any links here. But there’s information out there.