Posh performed in "cyber punk" outfit on Sunday
She made the revelation in an extract from her forthcoming autobiography, Learning To Fly, which was published in the Mail on Sunday newspaper.
Beckham desribed her illness and said that for a time she was "obsessed" with her appearance.
"In the gym, instead of checking my posture or position, I was checking the size of my bottom, or to see if my double chin was getting any smaller," she wrote.
The singer also described her "paranoia" after threats on her own life and that of her husband, England captain David Beckham.
Those fears may have been heightened on Sunday when she was briefly pelted with fruit and vegetables at BBC Radio 1's One Big Sunday event in Leicester.
Some members of the 100,000-strong crowd did not appreciate her sneak preview of her latest single Not Such An Innocent Girl.
Beckham ignored the disruption and continued with her performance.
Dieting obsession
After her weight dropped following the birth of her son, Brooklyn, Beckham was led to deny accusations of anorexia in the press.
Now she has admitted that in the early days of the Spice Girls, when they were under pressure from their management to lose weight, dieting was a preoccupation.
Beckham said it was Geri Halliwell, who encouraged her and Mel C to go running and take liquid meal substitutes.
In the following years, Beckham began to binge eat - in one passage from her autobiography she describes eating 10 bowls of Frosties in one sitting.
Beckham is in the midst of a publicity campaign in an attempt to secure her first solo number one.
The singer's new single, is to be released on 17 September, the same day as Kylie Minogue's record, Can't Get You Out Of My Head.
This means a relentless round of interviews and performances for both women in order to give their song a sales edge.
The battle lines were drawn up last week as Beckham secured headlines for her fake lip piercing and the serialisation deal for Learning To Fly which her publisher said: "Aims to set the record straight on the controversies that surround her".
Ferguson 'vindictive'
Beckham landed what is thought to be the highest price paid for the exclusive serialisation of an autobiography.
The book also describes her husband's Manchester United boss Sir Alex Ferguson as "vindictive" and said he had never said more than "hello" to her in more than four years.
She claimed that Sir Alex "went berserk" when David Beckham asked for two extra days off following their wedding, telling him to report for training on time - with the reserves.
Beckham claims that even after these differences, she persuaded her husband to stay, something which will surprise fans who blame her for enticing her husband away from football.
"I can't blame fans for thinking: 'You horrible cow, you're trying to take David away'," Beckham said.
"But it's just not true."
Learning To Fly, complete with photographs of the star from spotty teenager to plump student to solo career snaps, will be published on 13 September.