Do your cats have a medical condition?If I didn't put my 14yr now 15yr old cat on senior she would be dead now I tried all kinds of food and she was getting worse and she is active and healthier now on her 11+ cat food and my 7yr now 8yr cat gets senior 7+food its 75% of his diet if I didn't do that he would be dead also.
Because here is discussion of protein requirements of senior cats, a cited/referenced piece by a small animal specialist endocrinologist: http://endocrinevet.blogspot.no/2011/11/optimal-protein-requirements-for-older.html
And here is research indicating that a high protein diet in cats is the best diet not only for weight loss, but for maintaining body weight after weight loss: http://jn.nutrition.org/content/139/5/855.full.pdf+html
By the same endocrinologist, a discussion of why high protein is so important for cats: http://endocrinevet.blogspot.no/sea...0-05:00&max-results=20&start=14&by-date=false
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To the OP: You may also want to consider giving your kitty 500mg of krill oil (I use NOW Neptune krill oil softgels) daily. I have a kitty that has arthritis (most of the ball joints of her hips are gone), and her mobility has dramatically improved with the addition of krill oil. Of course, weight loss will be the most important thing in getting him moving, but if he has arthritis, the krill oil should help.