This is Dr. Atkins response to this...

Another more serious concern is the impact this going back and forth can have on your metabolism. (See “The Wrong Way to Do Atkins.”) You may have heard people say, “I love doing Atkins because I can cheat on the weekends, then go back to Induction on Monday morning.” While this behavior pattern may work for the short term, it will probably backfire in more ways than one. It's likely that your metabolism will adapt at a certain point—in a sense, developing a tolerance. People who repeatedly regain weight and go back to Induction sometimes find that they do not experience the dramatic and easy weight loss they initially enjoyed. Add in the facts that none of us is getting any younger and that our metabolism's natural tendency is to slow down with passing years. Finally, your body pays a price healthwise if you dramatically switch back and forth repeatedly from a fat-burning to a glucose-burning metabolism.