When Jessica Simpson needed to slim down for her new movie, Major Movie Star, she turned to celebrity trainer Harley Pasternak. The result? The author of 5-Factor Diet and Fitness helped her shed over 20 pounds.

Click here for more details on how Pasternak helped Simpson change her eating habits.

For more on the menu and workout plan that's changed the bodies of stars like Katherine Heigl, Sophia Bush, Mandy Moore, and Vanessa Williams, check out the new issue of Us Weekly, on newsstands now.