The British heartthrob had to strip off to shoot a saucy scene with real-life girlfriend Kristen Stewart for Breaking Dawn: Part 1, which is due out next month (Nov11).
Pattinson threw himself into a pre-filming workout regime with vigour, but admits he couldn't wait to binge on food after the big scene had wrapped.
He tells 15a20.com, ""The body of the characters seem to be a big deal. So in this movie, I thought, 'OK, it's the last one, I'm getting a six pack,' and I tried, but it's very hard. I worked out so hard for six months, so after filming the sex scene, the first thing I thought was, 'Time to eat!'""
And the star admits he felt compelled to concentrate on getting a ripped chest so as not to be left looking puny next to his musclebound co-star Taylor Lautner.
He adds, ""You also have to worry about how you're going to look. I had never been so worried about that before, but after seeing how much Taylor works out for the movies, I thought, 'Well, everybody is really going to judge me now.' I always tried to think about positions where I could tense the muscles, and suck in my belly.""