Andrew H. Walker/FilmMagic
11/29/2011 at 11:00 AM EST
When Kelly Preston gave birth to her son Benjamin Travolta
a year ago, she knew losing the baby weight wouldn't be easy.
"When I got pregnant at 48, I didn't know if my body would bounce back," the actress tells PEOPLE.
Turns out, Preston had no need to worry. Not only did she shed the baby weight, but over the past year she's dipped below her pre-pregnancy weight – and is relishing a new figure and renewed vitality as she approaches the mid-century mark.
"I lost 39 lbs., have more energy than I had 20 years ago – and I feel amazing," Preston says. "I'm going to be turning 50 next year – holy s–––! Hey, 50 is the new 30."
So besides tennis and keeping up with an active baby, what's her secret? The Organic Liaison weight-loss program created by Preston and John Travolta's close friend Kirstie Alley
, who cleared her own hurdle this summer when she dropped 100 lbs
"I was so excited when my best friend created Rescue Me," says Preston of the diet plan's organic supplement system that targets cravings and "boosts natural energy," according to the Organic Liaison website
"It's pretty much the most genius thing ever," says Preston, who recently signed on as the weight-loss system's spokesperson.
But a rocking a new body is not the only cause for joy in the Travolta household.
"We just celebrated Ben's first birthday," she says. "I can't believe it's already been a year."