Due to the nature of our evolution as humans the body physically reacts to sudden changes in your diet. I the not too distant history, your body was able to perceive changes in eating habits, to know when to store extra fat, and to know when it was a time of famine. When you undertake a massive diet change, the body reacts and makes necessary changes to adapt to the new eating habits. I believe this is why a vast number of people do not have success in dieting. Not only are they battling with the changes to their old eating habits, they are also battling with their own body's reaction to the food intake and diet changes!