My Artful Dodger had Irritable Bowel Syndrome from about age 8 to 16, when he died of CRF, Cardiomyopathy and Hyperthyroidism - my absolute favorite cat and he had everything! Dodger's first symptoms were blood in the stool but he was treated initially for giardia, worms, etc. Later he became nauseated and just stopped eating. I had to force feed him for a while and then give him the Innovative Veterinary Diets, venison & field peas which worked very for a year or so. (Hills may be copying IVD's diets which have proved very popular.) The final diagnosis of IBS wasn't made until he had surgery and a biopsy of his small intestine was taken, there is no other way to diagnose this condition with any kind of accuracy and even then they say "it looks like". This kind of gastrointestinal problem is really hard to diagnose and treat, not just IBS but similar disorders. Dodger was given heartburn medication for years, and that did help him. There were months, even a few years at a time when everything would be fine and then there were flare-ups that just happened. Each time he decided that he didn't like one special diet and stopped eating altogether, I'd have to go through several more to find one that he'd eat and that wouldn't cause bloating, nausea and blood, slimey stools. Dodger's blood tests were normal also.

Good luck. It's much easier when they can find something with a blood test.