A few people have found this useful so I thought I'd post it here so it didn't get lost:
Here is some maths (sorry I like numbers)
I don't know your height and weight so I am going to invent a person, lets call her Annie.
Annie is 5'4" tall and is 5 stone over her ideal BMI (of 23) so weighs 14st7lbs.
She has a moderate exercise regime. To maintain her weight she needs to eat 2500 cals per day.
She wants to lose the weight as quickly as is healthy, according to caloriecount the recommended loss is 2lb per week. In order to do this on a normal calorie restriction diet Annie should eat 1444 cals per day (or 10108 cals per week)
If Annie wants to do 5:2 then she should eat 1821 calories on eating days and 500 cals on fast days. (so the same 10108 per week)
If Annie wants to do 4:3 then she should eat 2152 calories on eating days and 500 cals on fast days. (again the same 10108 per week)
These figures are for optimum weight loss. As Annie loses weight the figures can be reduced according to her new weight.
If Annie eats much less than these figures then her body is going to go into gluconeogenisis. Annies body can metabolise fat in all organs but 2, her red blood cells must have glucose, her brain needs some glucose but can survive with the rest of its energy coming from something called ketone bodies. The body can make limited amounts of glucose by breaking down stored fat. Ketone bodies are made from fat. However a body cannot make enough glucose from simply breaking down fat (if you are not eating enough) because the by products of the fat breakdown (fatty acids, ketones like acetone) simply overloads the blood. So the body starts to break down protein to make the glucose. This is starvation mode. This is when your body stops working properly. This is what Annie needs to avoid in order to stay healthy.
This is why the weekly calorie number is so important. If you are fasting on some days you need to give your body some fuel on the other days. Simply reducing the calories lower and lower will not help you lose weight, it will make you ill. It may also cause your skin to lose elasticity which means when you have lost the weight you will have excess skin. Steady, healthy weight loss avoids this problem.