Prince William acknowledges Princess Diana's struggle with bulimia for the first time

Prince William has opened up about his mother Princess Diana's struggles with bulimia for the first time.

In a new documentary from British broadcaster Channel Four, "Wasting Away: The Truth About Anorexia," the royal talks about the importance about speaking openly about eating disorders and other mental health issues. In recent years, William, along with his brother Prince Harry, have continued their late mother's efforts to de-stigmatize mental health issues.

The royal praised Mark Austin, a former anchor for a British news network, and his daughter Maddie, who is battling anorexia, for getting real about her struggles.

"We need to normalize the conversation about mental health. The fact that you are speaking out is incredibly brave," he said.

When asked by Austin whether he's proud of his mother for speaking out about her struggles, William said, "Absolutely. These are illnesses. Mental health needs to be taken as seriously as physical health."

If you recall, Diana revealed in a bombshell 1995 interview with BBC's Martin Bashir how her dramatic marriage to and divorce from Prince Charles had affected her self-confidence and mental health.

"I didn't like myself, I was ashamed because I couldn't cope with the pressures. I had bulimia for a number of years, and that's like a secret disease ... It's a repetitive pattern which is very destructive to yourself."

She continued: "It was a symptom of what was going on in my marriage. I was crying out for help, but giving the wrong signals, and people were using my bulimia as a coat on a hanger: They decided that was the problem -- Diana was unstable."

In the months and weeks leading up to Princess Di's 20th death anniversary, William and Harry have been opening up about their late mother. Most recently, Harry was featured on Telegraph's Mad World podcast, where he revealed his own struggles with depression following Diana's death:

"I can safely say that losing my mum at the age of 12, and therefore shutting down all of my emotions for the last 20 years, has had a quite serious effect on not only my personal life but my work as well," he said. "I have probably been very close to a complete breakdown on numerous occasions when all sorts of grief and sort of lies and misconceptions and everything are coming to you from every angle."

William and Harry's effort to open up about Diana's death is completely unprecedented, as members of the royal family have historically kept their personal lives out of the public eye. However, since starting their mental health charity Heads Together in 2016, the royal brothers have made the conscious decision to open up about their own personal struggles in order to tackle the stigma, raise awareness and provide help for people with mental health challenges.

For more on Princess Diana's 20th death anniversary, watch the video above.