Royal fans are wondering how a mother-of-three has such a tall and slender body physique, envying her long legs and tiny waist.

A few nights ago, Kate Middleton was seen in a gorgeous Alexander McQueen gown for the Royal Variety Performance, and many are wondering how she managed to keep her slim figure.

The Duchess of Cambridge reportedly wears a size six and has a waistline of 24 inches.

While she is known for her clean eating habits, she is also no stranger to the occasional diet trend. 

Now, Express has claimed that the Duchess of Cambridge went on a Dukan Diet before her royal wedding and after her pregnancy to shed some extra weight. Her mother is also on this diet.

Loved not only by Kate but also by Jennifer Lopez and Gisele Bundchen, the Dukan DIet incorporates 100 approved foods like non-starchy vegetables, lean protein, fruit and whole grain bread.

It was created to be as simple and effective as possible without having to say "no" to indulges from time to time.

While Kate was on this diet, she reportedly ate high-protein and low-carbohydrate meals that encourage muscle strength and minimizes fat. The Dukan Diet has different stages that require the dieter to eat different food.

In the first phase, Kate ate foods high in protein.

In the second phase, the duchess switched off between high-protein foods and the approved list of vegetables. When she was pregnant with Prince George, Kate enjoyed eating spicy curry -- a homemade vegetable dished cooked by an Indian couple near her Berkshire home.

In the third phase, Kate mixed the foods from the first two phases while incorporating two slices of whole-grain bread a day, a cup of fruit, small amounts of cheese, starches, roasted lamb and some celebration meals.

In the fourth phase, she implemented the first three phases to ensure that her diet is effective over time.

After giving birth, Kate reportedly was tucked into foods like avocadoes, berries and porridge to stay in shape.

To make sure what ingredients are on her food, it was also reported that the duchess likes to cook meals from scratch at home. When she is not on the Dukan Diet, she follows a strict healthy meal plan.

In the morning and afternoon, according to the Independent, Kate drinks smoothies containing spirulina, kale, matcha, spinach, lettuce, cilantro and blueberries.

For lunch, reports confirm that Kate follows an "organic diet" that is rich in antioxidant -- salads and bowls of seasonal fruits appear regularly on her plate.

When it comes to family dinners, Kate's go-to dish is a roast chicken.

A royal correspondent said that Kate Middleton indulges her hobby of cooking Prince William's favorite supper, roast chicken. "It's their idea of the perfect night."

As for treats, Kate has only one vice. According to Vanity Fair, she always drinks a glass of white wine in the evenings.

READ MORE: Kate Middleton Makes Her Kids Follow this One Strict Rule