Weight Loss theory

Today in class we watched a TED video and this lady was talking about a weight range. This is where our body believes we should be in a certain weight range and no matter what it will try to keep us in there. That way if you lose 20 lbs your body will try to gain it back. Which is why you have to maintain healthy habits. This is an evolutionary change in our bodies. Because food used to be scarce our bodies did what it could to maintain weight. The sucky thing is that the range can increase (say you gain 30 lbs and stay that weight for a year or so the range will go up) but the range does not easily decrease. Thoughts?