"The hormone that tells us we are full also regulates our desire for certain foods, researchers said on Thursday, in a finding that sheds light on why people gain weight and could lead to new treatments for obesity."

http://news.yahoo.com/s/nm/20070809/sc_nm/obesity_leptin_dc;_ylt=AsWZEOnTBz_ZDPzlDqhk9tciANEA