We can confirm that this diet does pack on pounds.

Charlize Theron is no stranger to body transformations -- remember her Oscar-winning role in "Monster"? -- and she did it again for "Tully."

Theron plays mother-of-three Marlo in the comedy directed by Jason Reitman and written by Diablo Cody. Right after giving birth to her third child, stressed-out Marlo agrees to hire a night nanny named Tully (Mackenzie Davis). Unlike in a Lifetime movie, Tully doesn't try to kill the family, she really does seem to understand Marlo's depression and helps.

Theron told Entertainment Tonight she gained almost 50 pounds for "Tully," and it "was hell" to lose it all over the course of a year-and-a-half.

Theron usually keeps to a pretty healthy diet, so she told ET this was the first time she was eating a lot of processed foods and drank way too much sugar. She did it to feel how her character felt and get into her mindset. "I was not that fun to be around on this film."

Here's how she gained the weight:

"The first three weeks are always fun because you're just like a kid in a candy store. So it was fun to go and have breakfast at In-N-Out and have two milkshakes. And then after three weeks, it's not fun anymore. Like, all of a sudden you're just done eating that amount and then it becomes a job. I remember having to set my alarm in the middle of the night in order to just maintain [the weight]. I would literally wake up at 2 in the morning and I'd have a cup of cold macaroni and cheese just next to me. I would wake up and I would just eat it...I would just, like, shove it in my throat. It's hard to maintain that weight."

Is it? Is it hard to maintain that weight? Seems easy from here.

But even for Charlize Theron -- who still looks like a supermodel at 42 -- losing that weight was rough:

"There's nothing fun about that. It took about a year and a half. It was a long journey, very long journey. And it's hard because I had press junkets and movies around it and nobody knew that I had done it for this. And I think in the beginning everybody thought I was wearing, like -- when the first photos came out, everyone [thought it] was, like, prosthetics. And then I went on press junkets and it was like nobody knew."

(Maybe that's why Anne Hathaway posted a video telling people in advance that she's currently gaining weight for a movie. Best to let people know in advance when you're Pulling a Bale.)

Theron said it took so long to lose the weight, she got worried.

"I was like, 'This is taking a really long time.' Because on 'Monster,' I just didn't snack for five days and I was fine. You know your body at 27 is a little different than your body at 43, and my doctor made sure to make me very aware of that. Like, you are 42, calm down, you're not dying, all good."

Theron has two children of her own, both adopted, and you can watch her ET video to see her almost cry talking about how much they've changed her life.

"Tully" is scheduled to open in theaters May 4. Check out the trailer.

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