There was a run of fat replacement back (iirc) in the late nineties. Olestra was one of the name brands.

It wouldn't digest at all, and it also wouldn't mix in happily with the rest of the body waste in the colon. Hence, anal leakage becoming a phrase you would see on food labels.

And you would, sometimes, have not only leakage, but diarrhea. Sometimes violent diarrhea.

Basically, the oil was slippery enough to escape the anus no matter how tight it was. And there was a lot of it, under pressure from other waste behind it.