Matthew McConaughey admits he has a lot less energy since his dramatic weight loss.

The 43-year-old actor - who sparked concern after he recently dropped 38 pounds from his already lean frame for his role as an AIDS patient in the new movie The Dallas Buyer's Club - revealed his grueling diet has taken a toll and he can't wait to eat a cheeseburger.

"'Overall, [I've] probably got 35 percent less energy, but there's been plateaus. Getting past 170 was really hard, but then once you get [to] 167 the next seven come off easy. Got down to 143 and that's where I want to be. So, once you get past the plateau, your body seems to understand, 'OK, this is where we're leaving now, this is where we are' and so the energy rises. Your organs [and] muscles shrink, your organs shrink [and] my stomach has shrunk as well," he said.

The actor also revealed that he lost most of the weight through dieting: "I'm doing cardio but I'll tell you what, the more I've learned is - and I think it comes with age too - is it's 90 percent diet. It's a matter of how much I eat or how little I eat."

He admits he can't wait to tuck into a gourmet burger once as soon as possible. "I will have some 70 percent beef, 30 percent fat ground beef, maybe a half-pound cheeseburger with another three types of cheese. I'm going to have buns with butter on both sides, toasted and grilled. And then I'm just going to sit back and let the [expletive] just drop on the ground," he said.