You would gain it back. If you have the time to rent/buy the book, it's a good read and will explain why -- this lifestyle is based on the glycemic index and going back to the foods that cane be included in a low cal diet (sugars, white pasta, white potatoes) will put back on what you lost.
Read the book, it may all click for you.
On top of which, there is no guarantee for the amount of weight-loss. Depending on what you need to lose, it may be 3 pounds but it's not all about the weight but changing the way you look at and think about food.
If you decide to read the book and want to try the beach, we're here for you!
Weight Lost Since January 3, 2015 via South Beach Diet