Taylor Swift has opened up about her struggle to overcome an eating disorder in Miss Americana, a new documentary about her life. The documentary premiered at the Sundance Film Festival, during which she said photographs and comments about her appearance had triggered the condition.

Swift said, in course of her 2015 world tour, under-eating left her feeling like she was going to pass out at the end of a show, or in the middle of it. The 30-year-old singer, also accepted she is not any way proud of it.

She added she struggled with the condition for several years. Some days, she would “starve a little bit and just stop eating”. The rest of the time, she kept lists of everything she ate and exercised constantly until she was a size double-zero (size two in the UK). But she denied having a problem when people confronted her about her weight.

The star says she now practises positive thinking when she is tempted to judge her body, consoling herself that it is better to think she looks fat than to look sick.

Miss Americana, which comes to Netflix on 31 January, received a standing ovation after its gala screening in Utah on Thursday night. Afterwards, director Lana Wilson praised Swift for being so candid about under-eating.