From Flab to Fab: Hollywood's Most Amazing Bikini Body Transformations

Trading in a chubby tummy for a rockin' bikini bod is no easy task.

With swimsuit season peeking around the corner, the question is, how do the stars slim down, shape up, and shed the pounds so that they can make a splash on California's ultra-competitive beaches?

New York nutritionist and Shape Magazine columnist Cynthia Sass gave celebrity site RadarOnline.com the lowdown on some of the biggest body transformations in Hollywood.

PHOTOS: Hollywood's Most Amazing Flab-to-Fab Transformations.

From Valerie Bertinelli's flab to fab figure to Heidi Klum's post-prego bounce back, see what Sass has to say about Hollywood's most amazing transformations.

More Amazing Transformations

BEFORE & AFTER: Drew Barrymore.

BEFORE & AFTER: Valerie Bertinelli.

BEFORE & AFTER: Kirstie Alley (the opposite way).

More Celeb Bikini Fun

PHOTOS: Cutest Hollywood bikini moms.

PHOTOS: Over-40 celebrity bikini bodies.

PHOTOS: Stars' favorite 2009 bikini styles.

PHOTOS: Full-figured stars' bikini bodies.

VIDEO: Sports Illustrated swimsuit model Bar Refaeli unveils her "little black bikini."