According to The Mirror, the star was afraid that she wouldn’t fit into the sexy clothes for her film The Dukes of Hazzard.
Simpson also worked out for two hours each day in the gym and running.
Jessica explained: "I went on the South Beach Diet for it."
The diet, created by a cardiologist from Miami, involves eating certain ‘correct’ carbohydrates and fats, to control blood sugar levels.
She said: "I trained for six days a week, two and a half hours a day. I hired a trainer and did lots of squats and lunges.
"I spent an hour a day running on the treadmill. I was determined to look good in those shorts."