Matthew McConaughey on 17kg weightloss

Los Angeles - 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 17kg from his already lean frame for his role as an Aids patient in the new movie The Dallas Buyer's Club - revealed his gruelling diet has taken a toll and he can't wait to eat a cheeseburger.

The father-of-two - whose wife Camilla Alves is pregnant with their third child, told Hitfix: "Overall, [I've] probably got 35% 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."

'It's 90% diet'

The actor also revealed that he lost most of the weight through dieting.

He said: "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% 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.

He said: "I will have some 70% beef, 30% 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."

See photos of a thin Matthew on set here.