Matthew McConaughey Explains Gaunt Appearance

USMagazine, Thursday, August 16, 2012, 3:30pm (PDT)

Matthew McConaughey is famous for his fit figure, but in recent weeks, he's been photographed looking surprisingly gaunt.

The reason, according to the 42-year-old newlywed, has nothing to do with vanity. McConaughey will soon play an HIV positive patient in The Dallas Buyer's Club, costarring two-time Academy Award winner Hilary Swank, 38.

PHOTOS: Matthew McConaughey's hottest shirtless shots

"I'm playing a guy who was sick and would have loved to have been healthier but wasn't. For me, it's more of a mental thing than a physical thing," McConaughey told Us Weekly at the NYLON Guys and Macy's INC party in Santa Monica, Calif. August 15. "I'll get down to the weight I need to get to. I'm on my way, and it's what I need to do for the job."

PHOTOS: Stars' favorite healthy snacks

McConaughey is eating "small amounts" of high-protein, low-carbohydrate foods to slim down. "It's only five ounces," he said. "That's the toughest thing."

As for his workouts, "it's mostly cardio," McConaughey told Us. "There's very, very little resistance." When filming wraps, the Texas native is most excited to chow down on a "cheeseburger."

His movie prep aside, McConaughey is also getting ready for the birth of his third child with wife Camila Alves, whom he married on June 9. (They have two children, Levi, 4, and Vida, 2.)

PHOTOS: Hollywood's most talked-about bodies

"The kids understand what's in her belly. Levi's very excited about it and we're looking forward to having a third [child] in the family," the actor said.

PHOTOS: Hollywood's hottest married couples

To help Alves, 30, during her pregnancy, McConaughey told Us he's "doing a lot of cooking! There's a Brazilian dish that her mother does, so we're having it more often than we're used to. That involves a lot of chicken, a lot of cheese and things like that, which I'm not having, but she is. She loves a good steak, so I've been making a good rib-eye for her when I can."

This article originally appeared on Usmagazine.com: Matthew McConaughey Explains Gaunt Appearance