Dieting is a big downside of being one of the world’s biggest action stars, says Matt Damon, who stars in Sony’s upcoming sci-fi actioner “Elysium.”

“I was grumpy on the set, because I was on a diet for the whole movie,” Damon admits at the film’s L.A. premiere Aug. 7 in Westwood. “At my age, I want to eat everything in my path.”

Damon walked the gray carpet for the premiere of the MRC-QED co-production, along with Jodie Foster, Sharlto Copley, Alice Braga, Diego Luna and the film’s writer-director Neill Blomkamp, who admits having a visual-effects background helps him make these films for a relatively low budget.

“But also I think it’s because my style is cheap,” Blomkamp jokes.

Most of the film’s cast, including Foster, were drawn to “Elysium” because of the involvement of Blomkamp, who wrote and directed the Oscar-nominated “District 9” in 2009.

“I thought ‘District 9’ was probably the most perfect movie that I had seen in a really long time, and I just really wanted to be in his next movie,” Foster says, but the actress also notes that the film’s social message was a major draw for her.

“The film has such a strong sociopolitical murmur, and it has such a strong insight into the ‘haves and have-nots,’ ” Foster says.

After party was held at the W Hotel.