Princess Diana lost so much weight between her wedding and honeymoon that her wedding ring had to be

• In her later years, Princess Diana opened up about her struggles with an eating disorder.
• She previously told a royal biographer she began experiencing bulimia following her engagement.
• A new royal book claims that by the time of her honeymoon, Diana's wedding ring was too big for her.

Princess Diana's struggle with bulimia took such a toll after her wedding that her ring had to be adjusted following her honeymoon, according to a new book.

Valentine Low, a Times of London royal correspondent, authored "Courtiers: The Hidden Power Behind the Crown," which was released in the UK on Thursday. In the book, he traces the modern history of the royal family through the lens of courtiers, a term used for the people working within each royal household. 

In the chapter "Growing Up," Low covers the experience of Michael Colborne, a private secretary who worked for Charles and Diana early into their marriage. Colborne, the author wrote, witnessed not only the "disintegration" of the couple's marriage, which began after their 1981 wedding, but the beginning of Diana's struggle with bulimia nervosa, an eating disorder.

According to Low, Charles had summoned Colborne to Balmoral, Scotland, where he and Diana spent some time following their tumultuous two-week honeymoon in 1981. It was there that Low wrote Diana opened up to the private secretary about how unhappy she was and, after an argument between her and Charles, that the prince threw her wedding ring at him. 

"Colborne heard Charles and Diana having a massive row. Then Charles suddenly appeared, and threw something at Colborne: it was Diana's wedding ring, which Colborne somehow managed to catch," the author wrote.

"Diana had lost so much weight that it no longer fitted and needed to be adjusted," he added.

Then 20 years old, Diana had already shown signs of her condition for months. The UK's National Health Service describes it as an eating disorder and mental health condition which involves the individual eating "a lot of food in a very short amount of time (binge eating) and then make themselves sick" in order to prevent weight gain.

According to Andrew Morton's 1992 biography, based on confession tapes recorded by the princess, Diana's condition began before her wedding to Charles.

"The bulimia started the week after we got engaged and would take nearly a decade to overcome," she said on the tapes, according to Vogue. "My husband put his hand on my waistline and said: 'Oh, a bit chubby here, aren't we?' and that triggered off something in me."

Her wedding ring is also not the only item to have been sized down due to her struggle with bulimia. As Vogue also reported, her wedding dress designer said when they began fittings for the big day, Diana's waist measured between 26 to 27 inches. But by her wedding in July 1981, her waist dropped to 23 inches. 

Later in life, the princess used her experience to become an advocate for others, according to Elle Magazine.

During a keynote address in 1993, she opened up public discourse surrounding eating disorders and touched on how they can affect an individual's overall health. 

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