The actor, who played wolfman Paul in the Twilight sequel, is almost unrecognisable as Nicaragua in gritty new film The City of Gardens.
Meraz had to shed his musclebound look and lose almost a quarter of his bodyweight to play the surfer in director Camilo Vila's new film, which is based on a real story.
The actor tells EOnline.com, "I didn't want to look like a Ken doll, you know what I mean?. It's a Peruvian prison in the '80s, which is nothing like an American prison. There's not much food. There are definitely no free weights to work out with. So I couldn't have a gym body."
Meraz stopped lifting weights, reduced his meal portions and even went on a juice diet while he was shooting.
He adds, "I could see my heart beat through my chest."
Meraz admits he's now trying to pack on the pounds again for the Twilight franchise's split Breaking Dawn films.