Beyoncé Admits: 'I Have Struggled Since a Young Age With Diets'

— -- Beyoncé is opening up how she stays in such great shape.

The singer sticks to a plant-based diet outlined by exercise physiologist Marco Borges in his new book "The 22-Day Revolution."

"I am not naturally the thinnest woman," she said on "Good Morning America. "And I have struggled since a young age with diets. And finding something that actually works and actually keeps the weight off has been difficult for me."

The plan is based on the theory that it takes 21 days to make or break a habit. Borges, who created a meal delivery system with the singer, advises clients to go vegan and avoid gluten, alcohol and soy for 22 days. Weight loss is a byproduct of the plan, but the main focus is to improve one's health. According to the performer, there are other perks as well.

"I felt like my skin was really firm, a lot tighter than when I deprived myself of food," she said. "[And, I] got the weight off fast, and the weight stayed off."