David and Rebecca have always struggled with being overweight.

The fraternal twins from North Prairie, Wis., had a tough time growing up and they turned to food.

On their 23 rd birthday, fitness trainer and "Extreme Weight Loss" host Chris Powell decided to help them lose weight. David tipped the scales at 470 pounds, and his sister weighed 398.

Click HERE to read Chris Powell's summer slimdown tips.

Powell brought David and Rebecca to Los Angeles to begin their transformation. First, they had to go to a loading dock, where their initial weigh-in was done on a freight scale.

What followed was a full night boot camp with a Navy SEAL team, followed by grueling workouts.

In an appearance on "Good Morning America" today, Powell explained how he approached helping the twins. He said he realized that they had a history of enabling each other's poor decisions - especially about their weight - so he separated them for the first 90 days of the program.

During that time, Powell trained with David, while his wife, Heidi, trained with Rebecca.

The twins' weight loss journey, and their final weight loss totals, will be revealed during the show's season premiere 8 p.m. ET today on ABC.

RECIPE: Chris Powell's Protein Waffle

RECIPE: Chris Powell's Chicken Tostadas

RECIPE: Chris Powell's Asian Wraps