There are some great posts in the low carb war zone about this subject. Some great info from the forum founders who really know their stuff! What I gleaned out of it is that it takes fat to burn fat. So when you are eating a lc diet, instead of a gram of fat being equal to 9 calories, it's the equivalent of 5 calories. So if you eat 150 grams of fat, normally that would be 1350 calories. But instead, because your body uses fat to process fat, it turns out to be about 750 net calories. If you don't keep your calories high enough, your body thinks it is starving and shuts down.

Now, when I started, I didn't know how it worked and I tried to determine what made sense for me. I seem to lose when my calories are in the 1800-2100 range. When I start creeping up to 2400 or so, then I don't lose as fast. I haven't stayed at that high of calories for a long enough period to know if I would gain or not, though.

I think it also relates to your carb count. So if you are eating at 20 carbs you could probably have more calories, but if you are eating at 40 carbs, you'd want less. But that's just my opinion - I don't have any facts to back it up.

Good luck!