Here is a spooky blog post (that refers to a study) about restricting the diet too much:

Reduced metabolism/TDEE beyond expected from weight loss

At the 3 month mark, the VLC group was burning almost 500 calories less than expected. There was substantial recovery after 3 months of maintenance. I did 2 months of below 1000 per day and 1 month of not much over 1000. If my TDEE was reduced by close to 500 calories, then 1500 would be pretty close to maintenance. The big message from it is to stop being sedentary.