A lot of people make the mistake of using a "light" food to help control an animals weight. You don't need a light food (they usually have a lot of fillers to make an animal think its full, but they just end up pooping more). Do portion control on your regular food, its a lot safer and better for the animal.
Or.. have your cat get a tapeworm and make all the vets think he has cancer. (That actually happened to me.. ugh). He'll drop half his body weight in 4 months! HATE TAPEWORMS. Now I'm going to de-worm my indoor only cats 1x a year :/