Chris Pratt shows off his incredibly muscular body on Instagram. Credit: Courtesy Chris Pratt

Chris Pratt has undergone his third total body makeover in two years! After getting ripped for his role as a Navy SEAL in 2012's Zero Dark Thirty -- and packing on 60 pounds of fat to play an attorney in the upcoming Vince Vaughn comedy Delivery Man -- the shirtless 34-year-old actor unveiled his six-pack abs and chiseled chest via Instagram on Sunday, July 7. "Six months no beer," Pratt wrote in the caption. "Kinda douchey to post this but my brother made me."

Pratt has been bulking up for his role in Marvel Comics' Guardians of the Galaxy, as he's been cast in the lead role of Peter Quill/Star-Lord. The film also stars Zoe Saldana, John C. Reilly, Glenn Close and Benicio del Toro. "Chris has been getting back in shape," a source tells Us Weekly. "His training is intense. They liked his body in Zero Dark Thirty but they want this character to be lean -- very lean -- so he's eating a lot less and doing 'two-a-days' as they call them, workout-wise."

The source adds, "Chris really put his body through a lot. He went from Zero Dark Thirty to putting on 60 pounds for his role in Delivery Man."

Pratt -- dad to son Jack, 10 months, with wife Anna Faris, 36 -- spoke about his weight gain for Delivery Man during a December 2012 appearance on The Tonight Show With Jay Leno. "I gained 60 pounds in about four and a half months. I just did it the old fashioned way: eating and drinking my face off," he revealed. "I've lost a little bit of it, but it was hard. It was probably just as hard as losing weight. You wouldn't believe it, but it is. It's these same principle: It's all about pain."