Matthew McConaughey** -- or what's left of him -- is perfectly healthy, thank you very much.

The 42-year-old actor is currently filming The Dallas Buyer's Club, in which he plays Ron Woodruff, a real-life drug addict in the 1980s who contracted HIV and became one of the first-ever AIDS activists. To get into character for the role, McConaughey has been dieting down his muscular frame to skin and bones.

"I'm a little more streamlined," he told the Daily Beast on Nov. 27 of his nearly 40-pound weight loss.

That's an understatement. The usually buff Magic Mike star -- now just 143 pounds -- has set the Internet abuzz with worry in recent months thanks to his gaunt, frail appearance. But he insists there's no cause for concern.

"I feel fine. I really do," he told the site. "I got the means to lose the weight in a really healthy way. I'm eating fresh fish. I'm just eating small amounts. I'm not being starved...I'm taking care of myself."

"I've been checked. My levels are fine. I'm as healthy as can be," he added. "The real health challenge is when you put [the weight] back on...You can't just start eating cheeseburgers and ice cream. Your body will go into shock, and it just won't work."

Learning about how his body does work has been something of a "spiritual journey" for the soon-to-be father-of-three. (McConaughey and wife Camila Alves** have two kids, Levi, 4, and Vida, 2, with a third on the way: The pregnant model, 30, is currently in her third trimester.)

"It's really been enlightening to understand my anatomy," he said. "I don't have the leverage I used to. I have body soreness."

That said, the physical changes are just one part of the actor's overall transformation.

"I'm playing a guy who was sick and would have loved to have been healthier but wasn't," he told Us Weekly in August. "For me, it's more of a mental thing than a physical thing."

This article originally appeared on Usmagazine.com: Gaunt Matthew McConaughey: "I'm Not Being Starved" for Movie Role