@SarahGen-- Because even though the metabolism slows down, the body still burns fat or muscle for its energy needs. So weight loss does occur. Some people also use starvation for quick weight loss to meet a deadline.
But this is not at all healthy. When an individual starts eating normally again, he or she will regain the weight very quickly. After the body experiences starvation, it wants to take precaution thinking that starvation may occur again. So much of the calories is stored as fat.