However, Fox's trainer Harley Pasternak - who also works with Lady Gaga and Robert Pattinson - insists the star's thin figure is down to genetics and not drastic dieting.
He tells Hollywoodlife.com, "Do I worry about Megan getting too skinny? Yeah! Believe it or not she has this really fast metabolism. So her training is all about body toning and sculpting. With her its really about she's eating enough of the right things. People thought, 'Oh she must be anorexic' but she's far from that!
"She gets frustrated at times because she's like 'I need to put on weight.' So she actually weight-trains to keep on that lean muscle tissue."