First, I hope this is for your cat. lol. Just kidding.
The best thing you can do is to educate yourself on what is good, not by listening to the manufacturers of cat food. They are selling what they want you to buy. They are profit motivated, not health motivated.
There have been some great studies done on cats and dogs in regard to their overall health and well being based on various diets. One of the most revealing was done by a Dr. Pottinger.
In his experiments he showed that all four generations of the raw meat and raw milk groups remained healthy throughout their normal lifespans. The first generation of all three processed food groups developed diseases and illnesses near the end of their lives. The second generation of all three processed food groups developed diseases and illnesses in the middle of their lives. The third generation of all three processed food groups developed diseases and illnesses in the beginning of their lives and many died before six months of age. There was no fourth generation in any of the three processed food groups. Either the third generation parents were sterile or the fourth generation cats died before birth! Remember, all four generations of the raw food groups were healthy throughout their normal lifespans.
Please note that the raw meat fed to these animals was not like the garbage sold in commercial so called "super" markets of today. The meat was derived from animals raised on grass and free range, not some feed lot like the Angus Beef cattle are today where they feed them grain from chemically treated soils.
Another study that is not been well publicized was one that showed a group of dogs that normally had life spans of about 6 - 8 years were fed diets that resembled the Pottinger experiments with lots of raw meat, etc. showed the dogs living to the incredible ages of 24 - 25 years old!
The commercial cat and dog foods of today contain lots of problematic foods for these animals. Many contain soy that is really bad for them, etc.
I suggest you combine your search for the weight loss diet to just looking for a diet that will net them long life and the weight should just come off and they will become healthy.
good luck

Switch to Dick Van Pattens Natural Balance Reduced Calorie Formula and reduce the amount you feed him.