The 31-year-old, who won an Oscar for her performance as a troubled ballerina in the 2010 movie, credits her trainer, Mary Helen Bowers, with getting her into shape and helping her learn complicated dance moves, reported Daily Star.
Bowers has now written 'Ballet Beautiful' to help others gain the grace of a ballerina and Portman has written a tribute to her friend in the book.
"My posture looked like I'd swallowed a broom... I had the same body since I was 14. With Mary Helen, it's completely transformed. I gained length, strength and tone. Arms and legs changed shape.
"In awe of dancers' bodies - definition in their arms, strong legs and beautiful line extending from fingertips to toes... Plus her healthy diet in conjunction with the exercises achieved our fitness goals," Portman wrote.
