The 29-year-old socialite - who showed off a larger, more muscular figure when she walked down the catwalk at a Brazilian fashion show last week - is said to have been inspired by her fitness fanatic boyfriend Cy Waits and is apparently now on a 3,500 calories a day diet to keep her energy levels up for her intense workouts.
A friend of the blonde star told Grazia magazine: "Paris has always preferred to stick to dieting all the time to maintain her figure. But since she`s been dating Cy, she`s got the bug. It`s a massive turnaround and now she`s working out so much that she`s often been seen on the treadmill drinking a milkshake."
Paris` new daily exercise routine - which includes weightlifting, bench presses and running on the beach with Cy, 35 - means she can now indulge in her favourite fast food treats after years of depriving herself.
Her friend said: "Paris has always loved junk food, but before she started working out to this extent, she had to deny her cravings. Now she feels she can indulge once in a while, as she`s burning so many calories."
Paris is delighted with the changes in her body and is planning to keep up her new regime.
The source said: "She wants to change her image and get properly healthy. At first she was unsure and it took some getting used to, but now Paris loves her new body. And Cy loves it too - he`s always talking about how hot and gorgeous she is."