During WWII the US government wanted to find out what was the best way to treat their servicemen that were prisoners of war who experienced starvation. To accomplish this they recruited conscientious objectors and underfed them until they lost 20% of their weight. They then re-fed these men until they regained their original weight. What they found was that they had a 40%increase in their body fat content.
Several decades later a group of people entered the Biosphere II in Arizona to live for 2 years solely on the food that they could produce themselves. Unfortunately, they could not produce enough and thus had significantly weight loss. When they left the biosphere and regained their weight their body fat content also was increased to the same degree as the conscientious objectors.
What these experiments show us is that the body has the ability to adapt to our environment. In this particular case the body increased it body fat stores so that it would be prepared for starvation should it happen again. This is also a warning to those who fast excessively to lose weight quickly.