Mainstream vet medicine often puts cats with IBS/IBD symptoms on manufactured hypoallergenic foods like Hill's Z/D, but these odds are often high in carbohydrates (fattening) and low in quality protein.. And according to some like myself and DVM, these food do not always work either - at least not for long.
For mild cases of IBD, grain -free canned diets are helpful.
For tougher cases, a raw diet of ground meat with cat-appropriate vitamin and essential-fatty acid supplements mixed in is a complete cure ( my opinion.)
I have seen cats ( two of mine) that have done well with food that have a lot of ingredients, even fruits and vegetables.
My first choice of natural cat food with the most simple, digestive-friendly formulas are:
Hounds & Gatos, canned. One of the higher quality choices out there. Several different meat formulas.
Pure Vita canned. Simple ingredients, grain free and low carb.
Tiki Cat Koolina Lua and Puka Puka Luau canned. Very simple recipes, but the only option is chicken.