I spent many years on <500 calorie/day diets. they DID effect my basal metabolic rate.

Because of that history, though, I lose best at about 1200-1300 calories/day, though my current body weight is 170. If I raise the calories, even keeping the carbs super-low, I gain. For people with similar histories, they may experience a similar limit.

However, if you haven't spent oodles of time on very low calorie diets, do NOT start now! I'm living, breathing proof that all it does is mess you up. I'll have to eat 1200 calories/day forever to maintain, too...don't let yourself get there!