Luckily, it was for a movie role. But it had real-life repercussions for her body.
"I was so bad with the food and alcohol in Nashville," the actress, 37, tells Harper's Bazaar in its May issue about the two-month shoot for Love Don't Let Me Down, in which she plays an alcoholic country singer on a downward spiral.
"If you saw me naked [now] compared to what I looked like when I did Iron Man 2, when I was exercising every day – I'll get it back together, but I've never eaten so much fried food and white flour in my life, ever."
The upside of playing a musician – and having to learn the guitar – was getting pointers from her husband, Coldplay frontman Chris Martin. "He was so helpful, so sweet and encouraging," Paltrow says. "I mean, it's embarrassing to be playing in front of your husband, especially when he's in one of the world's most successful bands."
She adds: "He is so great because he's so supportive of me as an artist … I really lucked out with him. He's deeply lovely and special."
Rewards at HomePaltrow's two new movie roles come after she took time off to focus on her kids – daughter Apple, who will be 6 in May, and son Moses, who turned 4 on Thursday. While she missed the challenge of acting, she says home life has its own rewards – often, simple ones.
"When you're having dinner with your kids and your husband and someone says something funny or you're dying laughing because your 3-year-old made a fart joke, it doesn't matter what else is going on," she says. "That's real happiness."
There is the paparazzi to contend with – whose photos, Paltrow says, tend to reinforce misconceptions about her. The pics "probably made me look like a really grumpy person," she says. "But when you're the person they're taking the picture of, it's like, 'You just followed me and made my kid cry and you're going to make money off it? You want me to look at you and smile?' "
But she recognizes that she doesn't have it as bad as her ex, Brad Pitt, and his family. "I feel bad for them," she says. "They have it worse than anybody on the planet. I mean, we get hounded too, but not like that."