As Miley Cyrus left the Winsor Pilates studio in Hollywood on April 27, onlookers couldn’t believe that the pin-thin woman with rock-hard abs was once a chubby-cheeked tween idol. “She looks so skinny!” a passerby tells In Touch of the 19-year-old, who works out in private sessions at the studio of Pilates guru Mari Winsor nearly every days. “She looks like a completely different person.”

Indeed, Miley has been wasting away since she traded her beloved burgers and potato chips for salads, and embarked on an extreme fitness routine that would put even Madonna to shame. Although she insisted on Twitter that a gluten-free diet was to blame for her shocking weight loss, insiders say miley vowed to slim down for a Hawaiian vacation with her boyfriend, Liam Hemsworth, in December. She has now gone above and beyond her goals, having dropped 20 pounds from her already tiny frame, believes private trainer Thomas Johnson of Santa Monica Sports Medicine. “Miley is now underweight at 100 pounds,” Johnson tells In Touch of the 5’4″ star.

But she isn’t stopping. On top of her Pilates regimen, Miley has been jogging and biking with her dog, Floyd, around her Touluca lake, Calif., neighborhood, and even enlisted the help of celebrity trainer Harvey Pasternak to further whittle her waist. With so much physical activity and not enough eating, Johnson fears she’s gone overboard. “She is likely overtraining,” he explains, “which can increase her chances of depression and hypertension.”

While she tells friends she feels better than ever — “health is happiness,” she recently raved — perhaps she should remember that most men love curves. Relax and eat a pizza, Miley!

Miley Cyrus

Miley Cyrus

Miley Cyrus