How much does your kitty weigh in at and how old are they? Here's a previous link from a topic similar to yours that may have more information for diet foods. Just make sure you give it a month or two before you see any results, cats dont just drop the weight immediately.
http://www.catforum.com/viewtopic.php?t=174
http://www.catforum.com/viewtopic.php?t=174