On an episode of the "Dr. Oz Show" that aired today, Aniston, 45, talked about playing a character Claire Simmons in the movie, that deals with chronic pain and how she had to stop working out for a while to make the part convincing to the audience.
"It really was interesting when you stop working out how you just ... my serotonin levels went down. My stamina was shot. I was cranky. I was irritable," she said on the show in a clip obtained by US Weekly. "I found myself short. I was hungry like crazy."
She said the role made here realize how important fitness is in her life.
"[It's] important to our soul, not just being able to fit into skinny jeans," she added. "Just your state of mind, your soul, all of that."