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Talking to People, Anderson confessed that she struggled to watch Paltrow's body change as a result of a poor diet and lack of exercise.
"They wanted her to eat a lot of fried chicken. It was frustrating for me, but they wanted her to gain," she said.
"When I trained her for the first Iron Man movie, I took 30 pounds off her. And she's been consistent ever since."
Anderson went on to say that Paltrow immediately returned to her previous diet and exercise regimen after shooting the film and had no trouble losing the weight gained.
