Dressed in a stunning off-the-shoulder gown, Princess Mary stole the show as she attended the 75th birthday of Queen Margrethe's sister Princess Benedikte of Denmark in Copenhagen on April 29.
But it wasn't just her embellished evening dress that wowed the crowd – 47-year-old Mary had everybody talking about her toned arms, too!
It's believed that as well as embarking on a fitness mission, Mary is a big fan of the keto diet – the weight-loss regimen spruiked by stars including Kim Kardashian.
The diet works by dramatically cutting carbohydrates and boosting fat and protein intake.
"This is more about Mary being fit and healthy than anything else," our insider says.
"She is turning 50 in a few years and she wants to make sure she stays in fantastic shape as she gets older."
WATCH: Princess Mary and Prince Frederik of Denmark share a dance. Post continues...
Mary's husband Prince Frederik, 50, is certainly a fan – he looked smitten with his gorgeous wife at the birthday bash.
The Australian-born royal has been taking her dedication to wellness to the next level.
"Mary is totally committed to being in the best shape possible. She's always been slim, but lately she has been focused on building up her muscles, getting strong and looking toned," our insider reveals.
"Mary is used to running around after her four kids. Now, she's kicked things up a notch by working with a personal trainer several times a week. They do a lot of weight lifting as well as exercises like squats and lunges."
The source adds, "Mary can see all of her hard work is paying off and she's loving her new look, too!"