Found this at Sadly, No!:
PLAYBOY: In Fight Club Durden pees into the soup he’s serving and farts on the food. Do you know people who have done that?
PALAHNIUK: I know people who worked at the big hotels in downtown Portland, and yeah, they would tell stories like that. There was a kid in England — and very handsome, well-presented kid — who told me, “I work in an upscale restaurant in London, and we do things to celebrities’ food all the time.” I said, “Tell me one person.” He said, “I can’t because there are only two of these restaurants, and it’d be too easy to find me.” I wasn’t going to sign his book until he told me one person. So he sheepishly goes, “Margaret Thatcher has eaten my sperm.” I started laughing. As soon as I did, he got bold. He said, “At least five times.”