vet recommended high carb food for my cat with chronic diarreha but it's not helping

Hello!  I am asking for some advice from fellow cat lovers out there regarding cat bathroom issues. My (almost ten) cat has had diarrhea for months now. The stools are NOT extremely soft and formed into the right shape, but still sort of soft, and have a very bad smell. My vet did tests for diabetes, kidney function, and thyroid and everything was really good except elevated protein level in blood or urine. He was tested again this February and everything is about the same. My vet put him on Royal Canin gastrointestinal diet (both soft and hard food) in Dec.2015 and gave him antibiotics for a week to stop the diarrhea. Although it wasn't a total cure it helped improve the hardness of the stools some. He has been drinking more water, the vet says, cause he is trying to make up for water lost in the stools. It could be possible that someone in my family is feeding him something he shouldn't have, human food or milk. Of course, I told family more than once not to give him anything different but maybe they're not listening to me. I don't know what else the problem could be! Vet told me that a feces or blood test cannot determine if cat has had milk or not. The vet also said that he was put on a diet with lots of carbs because high protein could cause or worsen his diarrhea. Everything I've been reading about on what cats naturally eat goes against the high carb diet the vet recommended and it seems to me a high carb diet would cause diarreha or worsen it, but definitely not cure it. I think why he got the runs in the first place was because I was spoiling him with too many different kinds of treats and a new kind of canned food. I'm scared to start switching his diet cause he might get worse. I started him on the recommended dosage of lysine a few weeks ago and  I don't think it's making him any worse. Does anyone know about the kind of stools a cat would produce if having milk? Could worms be causing this? He hasn't been dewormed. What food would help stop the runs? What does a high protein level mean in a cat and is that why he should stay on low protein diet? Please, if anyone has some advice or experience with this could you please send some helpful info. my way.  Thank you

So, it doesn't sound like your vet ran a fecal test on your cat, is this correct?

Does your cat go outside? Any other animals in the house?

Was there at changes in your cat's diet back when the diarrhea started?

What food were you feeding (brands and flavors)?

Your vet is correct that there is not test that will tell you what a cat is eating.

Is there away to confine your cat to your bedroom so that you have full control over what your cat is eating and has access to?

My cat hasn't had a fecal sample done yet, but I'll consider getting that done soon so that a parasite problem can be ruled out. I used to tie this cat up outside for 20 min. a day and keep watch on him but for the last couple months I have been putting him on a harness and go outside with him. He did get out a couple of times and got in cat fights. He was in a cat fight a few weeks ago and doesn't appear to have any abscesses anywhere. I have one other cat in the house and he eats the same food and is fine.

I had been giving my cats new and extra treats and whiskas cat milk. The sick cat also had been given a bit of cooked chicken, fish or roast beef a few times a week, but this is nothing new. I didn't notice at the time that he had diarrhea, until weeks after I started giving him the new foods. I think this may have been why the diarrhea started but I don't know why it is still continuing. I gave him a couple Temptations cat treats various flavours daily and about 5 Temptations Dental Treats for his teeth, every day. I also gave him Greenies Dental Treats 5 a day when the bag of Dental Temptations ran out. I gave him Whiskas Cat Milk more often than usual, sometimes daily and gave him a new treat - Weruva Cat's in the Kitchen Pumpkin Jack Splash Pouches 2 or 3 times a week. Their staple diet was various flavors of canned Friskies cat food and Whiskas Dry cat food. I think I gave him another kind of food in a pouch for a while but can't remember the name right now. If the diarrhea doesn't stop it can develop into pancreas disease. I am so worried.

I cannot restrict him to a room that only I have access to.

So, first I would think that feeding your cats only one thing would help.  Cats have very sensitive digestive systems and you were feeding him a lot of very different treats and foods.  This could have been ok earlier, but maybe he can't handle it anymore.  I would assume this is a lot of the issue whether he is secretly getting people food or not.  He also does not need all of the treats to be happy, so don't worry about that.

I am wondering if your cat has IBD.  Has your vet mentioned that?

Is the doctor recommending that you stick with Royal Canin?  If so, I think wet and dry in one protein at a time would a great choice.  If he continues to have a reaction, you can slowly rule out one protein at a time.

Thanks for the advice. I just wonder if the high carb Royal Canin diet is actually keeping the diarrhea going and that a high protein, no carb diet would maybe fix the problem. He has been on just Royal Canin for two months now with no improvement, but you are right I should have a talk with my vet. Thanks for all your help.