Re:Calorie Deficit and BMR
Thanks for the question and answer, I was recently wondering the same thing - came across the answer on another site but it's good to have it confirmed.
The way I understand it is - if your BMR is (roughly!) 2100 calories and you consume that amount you will not gain or lose weight. But if you exercise you will burn a portion of those calories consumed and therefore be in a deficit and you will have lost weight that day. The rule of thumb appears to be not to consume greater than 500 calories under your BMR and not to lose more than a 1/2 kg (1 lb) per week.
There are 3500 calories in a lb of fat, so technically this means if you want to lose weight at a healthy rate you should aim to be in a 500 calorie deficit each day - either through diet, exercise or both!
Feel free to correct me :)