Tracy Anderson, who also helped Paltrow get in shape for her role in Iron Man 2, said that she cut down her training, ate whatever she wanted and gained weight.
Anderson told Us Weekly: "She loves to eat. She pigged out. She did gain weight. We took her training down from six days down to three days a week and she just ate whatever the hell she wanted.
"She ate all of the weight-gain culprits. She loves bread and she loves cheese. [But the weight's] all off [now]. To get her back to shape, we bumped [her workout] back to six days a week."
Paltrow has previously blogged on her website GOOP about eating fried chicken and ice cream sundaes on the film set.