Had to join the site to respond to this!
My gal is nearly 12 and I had her since she was 2 (previously with my auntie).
She was traumatised from a lot of stuff I won't go into, but suffice to say this cat was at first totally shut down, and later simply a ball of nerves for a year. She still is extremely jumpy, though much happier after a lot of working with her needs and creating routines that reassure and please her.
When she came to me she was horribly constipated and not eating much either. We tried enemas, lactulose etc. She didn't go, it was awful. Xrays showed her backed up. She was vomiting after eating and started getting stress induced cystitis when this happened too. I gave her the best quality high meat content food I could find at the time, which was Lilly's kitchen organic, with added oil. It didn't seem to help. I tried high quality german food but it was beef based, and this seems to make her vomit even when well. My gal also isn't a drinker so I was making her warm, brothy meals and feeding her with a spoon just to get fluid into her. (She was scared of the fountain I tried)
Eventually I was lucky at some point to find a vet whose poor cat had the same issue, who after thousands of pounds of investigations still had no real answers why. It just happens to some cats, unfortunately! It can also turn into megacolon if not managed, which means colectomy surgery - not great for you or the cat.
She told me her cat was currently stable having 1/4tsp miralax in his food twice a day and the right food - she recommended Specific Digestive Support wet cat food with water added. I tried this and it did really helped!!
10 years later these are the 2 things that if I can help it she never, ever goes without, plus a bladder supplement which also calms anxiety. Things have been largely stable, unless something makes her feel unwell and her eating slows down (e.g. nausea from chewing dracaena leaves last year!).
The worst hiccup was when there was a supply issue causing manufacturing problem at Specific, at one point during covid.
For months I was trying to find some of the Digestive Support food and I was feeding her everything else I could think of, and she was super stressed, not pooing enough, not eating enough and getting cystitis a LOT. Tue vet could only suggest getting her back on her usual food as soon as possible and treating the cystitis. I managed to get her through this time using Senior Specific wet food mixed with dehydrated dry Digestive Support, but it wasn't ideal as the dry has a different makeup and she clearly didn't feel as well on it.
Once I could get the wet DS stuff again things were great, such a relief!!!
The Specific Digestive support is great as it includes rice and psyllium (both very gentle soluble fibre) and electrolytes and nutirents to help your cat and their poo stay hydrated enough.
The vet said it's possible my gal will still develop megacolon at some point, and I have insurance that will cover the surgery, should she need it.
But the best thing I can do is to keep her on the formula that keeps her regular, her poos soft, and her nerves calm! Every 2 days is regular for her, but occasionally it's 3 days. Any longer and I get her to the vet for an xray just to be sure she's not backed up. At the vet's advice I make a note of every poop she does to make sure I don't miss anything. (The vet called her book 'The Log Log', I put a star on my google calendar!)
Please note though, it's not really ideal for your cat to only have 1 kind of food, as if it disappears it's really hard for them to adjust to the change.
So I would strongly suggest you:
A) Try the Specific Digestive Support wet food;
B) Start giving 1/4 tsp miralax twice daily in the food (use a 1/4 tsp measuring spoon to get the right dose). I buy it on Ebay when I find a good deal, as its a US import;
C) Start adding a bit of water to your cat's food (I use hot as it warms the food up a bit and makes it more appealing);
D) If your cat is better and becomes stable enough, add some other foods into the meals in small quantities, just to get them used to variety in case you ever have to change foods in the future;
E) Remove as many stressors from the home as possible, and get a feliway diffuser going. Make sure at least 2 litter trays are available and clean. (If cats feel insecure they will sometimes hold onto their poo due to the instinctive fear that it may alert rivals or predators!)
I really hope this is helpful. Unfortunately it's not a cheap fix, but it saves you the stress and expense of vet visits, and the upset of seeing your cat suffer.
Sorry it's so long but I thought this info might save you some time in the long run. I could have gone years longer if it wasn't for that one vet with a cat like mine, kindly passing on her wisdom!!
Good luck and let us know how things go! This is a more common and more serious problem than many people realise, and it would be great for people to know what works for you 😊💖🐈⬛