MEGAN Fox's fitness trainer Harley Pasternak has revealed the actress's frustrations with gaining weight.

Fox has been looking particularly thin lately, but her trainer Harley Pasternak assured HollywoodLife the actress eats a lot and he explains why she is so skinny.

"Do I worry about Megan getting too skinny? Yeah!” said Pasternak, who trains Fox and other celebrities like Robert Pattinson, Katy Perry and Amanda Seyfried.

"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!'"

Pasternak said Fox gets very upset when she hears people think she has an eating disorder, especially since she struggles to gain weight.

"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."