Hello, I'm Dr. Deb and will do my best to help you today.
I'm sorry for this concern about Fatcat.
When one of my patients presents with this particular symptom and the most common causes (such as diabetes, hyperthyroidism, kidney and liver disease) have been ruled out, then I'd want to consider Late onset Inflammatory Bowel Disease (IBD).
Bloodwork would be normal, in most cases. Even an ultrasound wouldn't necessarily pick it up; it would take biopsies to diagnose. Obviously, most owners don't want to put their older cats through this procedure, which is understandable.
Response to steroids can often be very useful in diagnosing IBD; if the patient is 100% better, then this is the presumptive diagnosis in my opinion. It may seem like a crude way to diagnose this condition, but it's an effective one. And, since diabetes has been ruled out with blood work, then I don't worry too much about the consequences of this drug since cats tolerate steroids better than any species I know.
There is also usually a dietary component to this problem as well. Feeding grain free may help in some cases but often you have to feed more hypoallergenic diets such as Z/D (from your vet), or Natural Ba***** *****mited Ingredient Diets, Nature's Variety Instincts line, Evo duck or venison, Nature's Variety Frozen Raw Medallions (I recommend that they be zapped in the microwave for 10-15 seconds on each side).
Sometimes this is a trial and error process---finding the food that your cat will eat as well as one that helps with the problem.
Other treatment options include;1. l-glutamine 500mg/day crushed in food which is available at health food stores or online.2. NuCat Senior which is a supplement which contains anti-oxidants.3. Vitamin D3 1,000 IU day.. but not calcitriol. This amount of Vit D3 is not dangerous to cats4. Probiotics such as Forti Flora which are also available on the internet or possibly at local pet/grain stores.Unfortuantely, cancer might also cause vomiting of this sort but will also be difficult to diagnose without extensive testing such as an endoscopy. If FatCat were my patient, I'd want to treat him as if he had IBD before jumping to this diagnosis.
I hope this helps and gives you options to consider. Deb