Still, Heigl, one of PEOPLE's 100 Most Beautiful in 2007, didn't get there without putting up a fight. "I hate dieting. I hate the whole thing!" she tells PEOPLE in its new issue.
A revealing love scene in Knocked Up gave her the motivation to hit the gym with L.A. trainer Pasternak, who is known for his intense, time-efficient workouts.
"Katherine had a beautiful body, but it had never been toned," says Pasternak, who has worked with Halle Berry, Jessica Simpson and Sheryl Crow.
Heigl, 28, met with him for cardio and circuit-training sessions of 25 to 50 minutes, up to five times a week for six weeks prior to shooting. On-set, she'd jump rope or lift dumbbells outside her trailer. All the while, she ate Pasternak's 5-Factor Diet, which delivered five high-protein, high-fiber meals a day.
Even with a scheduled weekly "cheat day," says Pasternak, Heigl soon looked "tighter and more toned."
As for the former gym-hater, Heigl says now, a year after filming, she still works out regularly. "I used to have back pain. Suddenly, my body is supporting itself the way it's supposed to," Heigl says. "I wake up and I go, 'My god, I can't believe this is my body!' "
Frazer Harrison / Getty
Typical Workout for Heigl: 5 minutes of jump-roping, jogging, or stair-climbing for a cardio warm-up.
A circuit of 4 sets each of the following:
25 dumbbell flies
25 low-body crunches
5-25 minutes of cardio running, elliptical training, stair-climbing or biking, with the last five minutes working as a cool-down period.
Sample Day's Meals:Breakfast: French toast with ricotta pillows
Mid-morning snack: A mixed-berry shake
Lunch: TexMex salad with chicken and black beans
Afternoon snack: Hummus crudite
Dinner: Sea Bass with brown rice and edamame
(Recipes are available in Pasternak's Book, The 5-Factor Diet)