major weight loss?

Rockgod

I understand that major weight loss in a short period of time is not good for the human body.
I also understand that major weight loss for someone that has a LOT of wieght to lose in the beginning months is mostly water weight.
I'm not trying to lose 15lbs in a week, but I am curious how these people on wieght loss reality shows can lose 15-20lbs a week for months at a time. After about two months, isn't a mojority of the water weight gone and what their losing is actually fat?
Isn't that an unhealthy wieght loss?
Just curious...