Comic actor Steve Martin has become a diet expert after shedding 25 pounds (11.3 kilograms) to make new movie Shopgirl.

Martin, who stars alongside Clare Danes in the romantic comedy, couldn't believe critics actually thought he had put on weight when his efforts to lose it had really paid off.

He says, "I didn't change what I ate, I just started eating smaller portions.

"And I cut out bread - that's the real killer because I was eating half a loaf before dinner.

"All you have to do is that - then you can drink all you want."