I'd like to take credit for this information I'm about to state, but I got this explanation from another forum I'm a part of. In my own words, a dogs body is not built to digest grains at all. A dog will eat grains when they're in the food you put in their bowl, but you would never find a dog eating grains by choice. Canines are obligate carnivores, meaning nothing but meat is needed in their diet to meat nutritional satisfaction.
Many dog food manufacturers use high protien grains in the food to up the percentage of protein in the food. These are the "high protein" dog foods that can cause issues, because the protein is not digested (nothing "grain" is actually digested in a dogs body, their bodies are not built to digest grains), I suppose this is more due to "lack of meat protein" than too much "grain protein". Maybe saying that they're "dangerous" was overkill, but "inappropriate for canines" would make more sense?
Anyways, this was a response from a member from another forum, she has a lot of knowledge with canine nutrition, and canine anatomy

very well respected member of that forum
"Dogs may scavenge leaving from humans, however if given a choice they will eat meat. No dog goes into a corn field or other grain field and chows down. Dogs do like to eat a few veggies & fruits on occasion, however they will eat meat instead if given the choice. Dogs have different enzymes in their digestive system then humans, they also have shorter digestive tracts. They aren't designed to process grains. Their teeth & jaws aren't designed for chewing, grinding grains & fibers, they are designed to rip & tear meats and chew on bones.
Just because dogs have been living around humans for years doesn't mean they have been changed genetically from how nature made them.
Any good study on dogs will give people enough knowledge to know dogs are canines, dogs need meats & bones, dogs use & process meats & bones, that they process the nutrition from meats & bones effectively but they do not from grains."
"I also wanted to mention also that over the past years as grains have increased in dog foods there has also been a huge increase in skin issues, excess shedding, some cancers, kidney/bladder stones, kidney disease, liver disease, dental issues, runny eyes, allergies, paw chewing, digestive issues,chronic ear infections among other issues.
Up until 20 -25 years ago most people still fed high meat diets especially in the country areas. .These measts came from the butcher, hunters,leftover from the family dinner (not as much fatty, deep fried,processed meats years ago) Also foods such as Purina and IAMS/Eukanuba contained a lot more meat and little grains. The "grain thing" has been a pretty recent additive and one that has been highly done by the big dog food companies..Nestle, Proctor & Gamble, Colgate/Palmolive, Mars, Del Monte....in order to cheapen the production of the food and mass produce it as cheaply as possible. None of this has done any favors to our dogs."
Hopefully this helps
