ILLINOIS - Few people want to be obese, but growing numbers are, and the problem is proving increasingly difficult to conquer. Obesity is obviously associated with taking in more calories than a person burns off. But fighting fat also means understanding how and why it develops, and two new studies have taken a giant step in coming up with some ideas. The studies, published in January 1 Genes and Development, have pinpointed a master switch that directs the formation of fat cells. They have also suggested a new strategy for treating obesity, and a way to improve on a class of widely prescribed diabetes drugs.