Megan Fox looks pretty thin in her recent ad campaign for Armani. But don't worry, says her trainer, Harley Pasternak, she definitely doesn't have an eating disorder. In an interview with, Pasternak reveals that the 'Jennifer's Body' actress really struggles with gaining weight.

"Do I worry about Megan getting too skinny? Yeah!" Pasternak said. "Believe it or not she has this really fast metabolism," Pasternak continued. "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!"

Fox gets upset when people question whether or not she's anorexic. "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," he said.

Pasternak, who also trains Robert Pattinson, Katy Perry and Amanda Seyfried, then described the kind of training he does with Fox that results in her toned abs. "The key to Megan's abs is first of all genetics," Pasternak admitted.