The 32-year-old actress, who has five-year-old daughter Suri with husband Tom Cruise, has apparently already lost a stone. At 5 ft 9 inches she weighs just eight stone.
A source told Now! magazine that friends fear she is taking it too far.
'Katie's been on an extreme diet. She started cutting back on food, particularly protein, to drop a few pounds as she'd seen pictures of herself looking a little heavier than usual.
'She's gone from a UK size-10 to a UK size-6 and she's still losing weight.
'Katie still feels inadequate when it comes to Hollywood.
'She thinks she's only where she is because she's married to Tom Cruise.
'Her view, however wrong, seems to be that getting super-thin is the only way to win more movie roles.'