Lots of food allergies involve digestive problems. One of my cats developed an intolerance to grain after having had no problem with the grain in ordinary dry cat food for over 10 years. He lost more than half his weight and had stools of yogurt consistency. Within less than 2 days of switching to a grain-free food (has potato replacing the grain) he had perfectly normal stools again.
This is common in people as well, e.g. celiac disease and similar conditions.
Other racial groups have a lower or unknown degree of difficulty.
There are Vietnamese who are so adapted to consuming rice they actually cannot handle any other grains. This is not the same problem folks with Celiac have ~ the genes involved are quite different.
True allergies are not the same as intolerances, which generally involves an inability to digest a food or a component of the food. Allergies are immune system reactions. Certainly, you could get some digestive issues with an allergy, but that is not generally the major symptom.
And, it puzzles me that cat food would have grains in them (not that I don’t believe you, I’m sure they do it to save money). Cats are true carnivours and cannot live without meat, unlike dogs which can (altho I don’t recommend it).