It doesn't have to be with anorexia. Any time when you are constantly depriving your body of sufficient nutrients can cause your period to stop, however it takes a bit of time to happen. However, it depends very much on the individual body. Some people can be very underweight, not eat emough etc but still have their period. Some people can be normal weight/overweight and lose their period.
Aaa i see how we have got our wires crossed. Yes if you go under 5/6 stone then you are likely to stop on a semi-permanent basis. But your period may also stop in a number of situations when u put ur body under high stress - e.g. if you suddenly stop eating.