Taylor Swift revealed that she previously struggled with an eating disorder as she discussed her "unhealthy" relationship with food.
The singer said she would "starve" herself if she saw a photo where she thought her stomach was too big, leaving her feeling as if she might pass out during or after shows on her 1989 tour in 2014.
She addresses her issues with eating in her new Netflix documentary, Miss Americana, which had its world premiere at the Sundance Film Festival.
"I didn't know if I was going to feel comfortable with talking about body image and talking about the stuff I've gone through in terms of how unhealthy that's been for me - my relationship with food and all that over the years," Swift said.
"But the way that Lana [Wilson, the film's director] tells the story, it really makes sense.
"I'm not as articulate as I should be about this topic, because there are so many people who could talk about it in a better way. But all I know is my own experience.
"And my relationship with food was exactly the same psychology that I applied to everything else in my life: If I was given a pat on the head, I registered that as good. If I was given a punishment, I registered that as bad.
"I remember how, when I was 18, that was the first time I was on the cover of a magazine.
"And the headline was like, 'Pregnant at 18?' And it was because I had worn something that made my lower stomach look not flat. So I just registered that as a punishment.
"And then I'd walk into a photoshoot and be in the dressing room, and somebody who worked at a magazine would say, 'Oh, wow, this is so amazing that you can fit into the sample sizes. Usually we have to make alterations to the dresses, but we can take them right off the runway and put them on you!'
"And I looked at that as a pat on the head. You register that enough times, and you just start to accommodate everything towards praise and punishment, including your own body.
"I think I've never really wanted to talk about that before, and I'm pretty uncomfortable talking about it now.
"But in the context of every other thing that I was doing or not doing in my life, I think it makes sense [to have it in the film]."
The film features a voiceover from Swift, according to the magazine, in which she says: "It's not good for me to see pictures of myself every day."
She added: "It's only happened a few times, and I'm not in any way proud of it".
But there have been times in the past when she has seen "a picture of me where I feel like I looked like my tummy was too big, or someone said I looked pregnant... and that'll just trigger me to starve a bit - just stop eating".