Anonymous wrote:OP, I don't like to second guess doctors, but this one really sounds like a quack.
First of all, almost every doctor will suggest that an aggressive weight loss goal is around 2 lbs per week. I get why you want to lose weight fast, but there's a reasonable limit. If you actually lost 2 lbs per week, you'd be back to a healthy BMI in around 8 months, which is a stellar turn-around.
Second, BMI isn't as important as body fat percentage, and if you lose weight that fast you're going to lose absurd amounts of muscle and keep fat around your midsection (not good if you are at risk for diabetes), which will cause rebounding and not improve your health. Going slower and adding strength training to keep muscle seems like a much better idea.
Finally, and perhaps most importantly: Its highly likely what the doctor is suggesting is literally impossible. A pound of fat has 3500 calories in it, and, at your height, your caloric needs are likely only 1800 calories per day. Work out the math on that. If you literally stop eating, you won't lose 4 lbs in a week (over a sustained period; you would the first week due to water loss).
I would get a second opinion.
Sorry, just to add, the high end of the range (800 calories) is significantly less crazy than the low end (500), and you'll ultimately need to be around there to keep up 2 lbs per week. But a 300 calorie difference is a huge difference when you're talking that big of a deficit.