The 29-year-old actress has apparently had to slash her daily food intake in a bid to drop two dress sizes in time for her death scenes in the movie adaptation of `Les Miserables`.
The `Devil Wears Prada` star - who plays single mother and prostitute Fantine - is said to be surviving on just two apples a day and a protein shake.
A source said: "Anne is playing a destitute factory worker-come-lady of the night. "Unfortunately, she only has 15 to 20 days to lose as much weight as possible - up to a stone-and-a-half (21lbs) - because she`s been filming the scenes showing her fit and healthy, almost slightly chubby.
"It`s not ideal but, as with any high-budget movie, there are strict time constraints."
However, the insider insists producers of the movie - which also stars Hugh Jackman and Russell Crowe - will be keeping a close eye on Anne`s health.
The source told the Daily Mirror: "Anne knows the risk of such an extreme diet and will undergo weekly medicals to make sure everything is still functioning as it should do."