I heard that you shouldn't go below your BMR because your body will go into starvation mode. your BMR is the amount of calories you burn, as Jenicra rightly said, when you do sweet F.A. Just keeping you alive your body burns that amount of calories! Sweet huh? So you can eat 1700 or whatever and will probably lose weight if you do a little exercise!
Oooh, also remember that the recommended daily calorie intake is 2000 calories, and that's based on a "normal" weight person (that's for weight maintenance). Being overweight means that you naturally burn off more calories in a day because your body is doing more work to keep you upright! So to maintain, you probably need about 2500 cals or maybe more! So you see 1700 isn't really a lot.