Chris Hemsworth posted a shirtless photo to Instagram Sunday and, at first glance, it's hard to tell that it's the typically buff actor named 2014's Sexiest Man Alive that we all know and love. His dramatic weight loss was necessary, though, for his upcoming role in "In the Heart of the Sea."

"Just tried a new diet/training program called 'Lost At Sea,'" he captioned the photo. "Wouldn't recommend it..."

In the action-adventure film, which is about the aftermath of the real-life maritime disaster that inspired "Moby Dick," Hemsworth stars as Owen Chase. In order to shoot the shipwrecked scenes, he had to drop a lot of weight to give off a disheveled, starved look.

"We have to shoot the really skinny stuff where we drop down to 5, 6, 700 calories a day, a good three or four weeks and it's going to be pretty uncomfortable, but we'll be together in our misery," he told "Entertainment Tonight." "I spend more time thinking about food than anything else at the moment."

Back in August, he opened up about losing the last 15 pounds for the role and how exhausting it was. "When you're already starting off lean, it's brutal to chew through that kind of weight," he told Entertainment Weekly. "Every pound feels like a kilogram.... We kind of went insane, weighing ourselves every day. We all felt like a bunch of supermodels, trying to get down in weight for a show, or something. That's all we spoke about. You've got 15 burly blokes on the sea and all we talked about was our diet, and who'd lost more weight, and who's looking really skinny. It's ridiculous!"

"They had to work out every day, even on shooting days," director Ron Howard told Entertainment Weekly. "Because they had to lose the weight pretty fast and they had to lose it safely. They needed to keep burning the calories and we also needed that sinewy strength that was more of that era, as opposed to a kind of cut, buff look."

"In the Heart of the Sea" hits theaters Dec. 11.