The heartthrob said he was determined to get buff for the scenes in Breaking Dawn – the two-part final installment of the vampire saga.
But he revealed that despite his initial enthusiasm, he found it impossible to keep up the good work.
He told Entertainment Weekly: 'When I start [getting fit], I just go crazy about it. It's like the only thing I can talk about to anyone. So I was like, 'I'm going to keep this up the whole time, so for whatever movie I do afterwards I'm going to be so buff'. And then literally one day after my last shirt-off scene I started being all, 'nom, nom, nom'. And I didn't realise I had one more [shirt-off] scene, and you can see it one of them - I'm clearly [sucking in].'