There's a good reason why Pippa Middleton always looks like she's in tip-top shape. Not only does she spend endless hours in the gym working on her fitness, but she also likes to stay on a strict diet when it comes to what she eats in the kitchen.

The British celebrity recently opened up about her controlled calories and how the Dukan diet has helped energize her with her very active lifestyle.

"I try to eat wholegrain, energy-fuelled carbohydrates and three meals a day," she told Hello! Magazine. "I also increase my portion sizes based on my training level, with lots of brown rice, lentils, quinoa and sweet potato, and for breakfast, porridge and rye toast."

The 32-year-old, who rose to fame at her sister Kate Middleton's royal wedding back in 2011, is also a hardcore triathlete who likes to participate in International competitions, like the Otillo Swim-Run championships, which is a grueling 47-mile race.

She added, "I don't get fussy, though, because I enjoy food too much. But a week or so before an event I'm quite conscious of eating really nutritious foods to store energy and I cut out alcohol, refined sugar and processed foods."

And even though Pippa is very strict with what she eats, she does allow herself the odd cheat day, in which she lets herself indulge in the better things in life like chocolate and wine.

As she explained to the magazine, "I grew up with the belief that good health is about moderation in all things, so I'll be celebrating healthy living through exercise, a balanced diet and a little of the naughty stuff sprinkled in--I'm talking chocolate, crisps, and the occasional glass of wine."

Tags: Pippa Middleton