The 'Toxic' singer joined the judging panel for the Fox talent show earlier this month along with Demi Lovato.
Ahead of the launch and first auditions, Spears allegedly started to avoid carbohydrates and begin a new exercise regime in order to get into shape.
"Britney has been on an extreme diet and exercise regime. She has been cutting out carbs and replacing meals with juice drinks," a source told The Sun.
"She was doing several hours of exercise a day before the launch and is trying to stick with the regime."
Spears dismissed reports that she walked out of the first day of X Factor USA auditions last week, saying that she is having the "best time" at the judges' table.
The X Factor USA returns later this year on Fox.
Photo gallery - Judges arrive for the X Factor auditions in Texas:
Copyright: WENN Arnold Wells