Kate Middleton's mom's 56-pound weight loss spurs popularity of Dukan Diet

Kate Middleton's mother, Carole Middleton, lost 56 pounds last year on the controversial high-protein, low-carb Dukan diet, catapulting sales of the French diet book in the U.K.

“I’ve been doing it for four days and I’ve lost four pounds,” the former British Airways flight attendant said in an interview last year.

Carole, 56, began the diet a month before her daughter Kate, 29, announced her engagement to England's Prince William, 28.

Since its UK launch in May 2010, the Dukan Diet book has sold over 250,000 copies in England, thanks in large part to Carole's indirect promotion.

The Dukan Diet will be released in the United States and Canada on April 19.