04/06/2011 at 06:00 PM EDT
Matthew Morrison is known for his ripped physique – but how does he do it?
star, 32, who shows off his washboard abs in a series of beach photos in a recent issue of Details
, also reveals a slightly unusual diet secret.
For three days before the photo shoot, the actor, who plays McKinley High's glee club leader Will Schuester, ate nothing but sweet potatoes.
"I had to have a six-pack on show," Morrison told the U.K.'s Sunday Mirror
. "The potato acts as a sponge and your body literally shrinks and gets ripped and tight."
But not to worry, the actor promises he has given up the spuds-only diet since then: "Afterwards I ordered two pizzas and pasta!" he says.