The 31-year-old appeared in racy scenes as stripper Nancy Callahan in the 2005 original, but is said to be less confident about her body since giving birth to two children.
An apparent inside source told Life & Style magazine: "She was crushed when filming began and she didn't have her original body back so she told producers she wanted a body double.
"Her body is incredible, but it's not good enough. She's cut out carbs and does the P90X Ab Ripper twice a day, but she still isn't happy.
"She has been chain-smoking lately to keep her appetite at bay. Jessica struggled with an eating disorder in her teens and when she is under pressure to look good for a part, she can slip back into extreme behaviour."
Alba will appear in the Sin City sequel alongside fellow returning cast members Mickey Rourke, Rosario Dawson and Michael Madsen. Clive Owen has also been tipped to reprise his role as Dwight McCarthy.
Directed by Frank Miller and Robert Rodriguez, Sin City: A Dame to Kill For is released on October 4, 2013.
Watch a trailer for Sin City below: