This was exactly my thought, and I now wonder whether it's possible to determine via experiment. So how do you give the information to the subjects but not have them think that the researchers know it.

A confederate who's a subject and just happens to gossip about the thing is one way -- if the researchers proceed to deny it, you might be able to split them into groups based on a low status confederate versus a high-status confederate, and a vehement denial vs a "that study hasn't been verified" vs a "that was an urban legend."

Or providing a status signal that it's better to have a "bad" heart -- having a high status researcher who says "sure, we may live less long, but there are all sorts of other benefits they're not telling us about"

It's really hard to separate the information from the humans passing on the information.