Natrius is right. The independent variable in this experiment is calories ingested, not weight loss. They're specifically testing for what happens when you eat

Natrius is right. The independent variable in this experiment is calories ingested, not weight loss. They're specifically testing for what happens when you eat fewer calories. On page six, they compare rats that are lean because of exercise with rats that are lean because of calorie restriction. "...only calorie restriction was able to slow down aging and increase maximal life span." Simply having the participants lose weight would not allow as much to be learned about what they are studying. Complex controls are paramount for these human studies.