Their digestive system just didn't evolve to handle grain or grain-based food like bread. Dogs can't eat grain either; despite being omnivores, they can't digest it properly, and even if they could, they can't convert it into sugars and store it for later use. (Cats, of course, can't eat grain because they're obligate carnivores.)



For that matter, HUMANS aren't really designed for grain either... grains have lectins, which are a type of protein that we can't digest. We normally digest proteins into amino acids where they are absorbed by our intestines. Since we can’t digest lectins, they pass through the wall of our gut undigested as complete proteins. This damages the gut, inflames our bodies, and makes them unable to absorb many of the good proteins that we get from other foods.



But that’s not all. The worst part is that our body’s immune system is at a high state of readiness around our gut. Now, when lectins pass through the gut as a complete, undigested protein, our body mistakes them for foreign invaders and attacks with the immune system. After repeated attacks from lectins, your body gets smart and makes antibodies to automatically attack these nasty proteins. It builds immunity. Problem is that sometimes, the lectin looks like normal body tissue, and you end up with an autoimmune disorder that attacks proteins that have a segment that looks like lectin. Some examples of proteins that your immune system might start attacking: Insulin (hello, diabetes) and Myelin Basic Protein (MBP) in your brain (hello, multiple sclerosis).



I could go on, but the TL;DR is that too much grain is bad for you.