The idea is that unless you average about 1200 calories a day (for women, according to mfp), it is unlikely that you will get the nutrients you need. If you did a 3 day fast and then 3 days of 2400 calories that would be healthier. That's one reason why I am trying to lose eating 1500 now. The other reason is that I lost more muscle than I hoped during my rapid weight loss in the first two months (Sept/Oct when I lost 32 pounds in 65 days). If you do the fast and then do low calorie days after, you run the risk of more losing lean body mass than you might like.