The 24-year-old actor is currently filming the final instalment in the Twilight movie franchise, which is due to be released in two parts.
He told UK magazine Red: 'At the beginning of this last Twilight movie, I was at the gym constantly because I had to have my shirt off all the time.
'I was obsessively working out and got into being really healthy and cycling everywhere. Then, as soon as I finished doing the 'shirt off' stuff, I just stopped dead, and then never went to the gym ever again.”
He added: “Now I can't bring myself to go.”