The body does adjust, as Nathan has said.
Also, if you don't eat enough fats, the body thinks it's starving and will make its own fat. This is why some persons who eat very little fat have high cholesterol.
What is supposed to be done is that the food intake has to change. Also, you're not really supposed to diet but you're supposed to change your eating habits.
Another thing to keep in mind is DNA. Some of us are prone to stay thinner than others, no matter what we do. And some will gain more weight.
I would think I would be genetically geared to be thin. I was always sick as a child. I weighed a whole 129lbs when we got married but hit a high of 261lbs last year. Over double my weight. I dropped to 241 just before the holidays but picked up 10lbs over the holidays and keep yo-yoing between 240s-250. So we're gonna give this diet a whirl.
Goldie Hawn was once asked how she stays so thin, and she answered that is was all in her DNA. At least she was honest.