Initially grain-free diets were born from the thought that grains were evil, the misconception that dogs were carnivores, and that some dogs are allergic to grains.
Evil Grains
When we think of grains, corn and wheat are the most common ones. They have been villainized as "fillers", as genetically modified and therefore bad for you, and l lo as an unnecessary additive that takes away from your dog's nutrition.
I'm not going to go down the rabbit-hole of GMO, but what I will say is that dogs are omnivores. Dogs coevolved alongside humans from the agriculture days some 10 to 15 THOUSAND years ago.
Along which this coevolution and symbiotic relationship came the genetic modification of the dog's gut enzymes! Many of the dog breeds we have today have an increased number of enzymes to digest starches. Starches are the complex and digestible carbohydrates in plants. The starches are higher in grains such as corn and wheat, compared to cellulose, which is non-digestible in the dog's gastrointestinal tract, which is higher in our leafy greens. Cellulose is digestible by bacteria in the gut of hindgut fermenters such as horses and rabbits, but this is part of what we would call insoluble fibre.
Dogs are omnivores
The fact that people mistake the dog's cousin, the wolf, as an immediate ancestor of the domestic dog negates that environmental factors that went into the domestication of dogs.
Early wolves around the early human settlements would be offered food in exchange for alerting humans to dangers in the environment. The more human friendly would then breed with other human friendly wolves nearby, slowly this behavioural change would also start to show enzymatic changes. As these early descendants consumed more human grain products, they evolved additional copies of the enzymes to digest starch.
This enzyme called amylase is found in species that use starch or grains as a form of energy from carbohydrate. Keeping in mind that glucose is needed for life and starch is a long chain of many glucose rings.
The Amylase 2B is the version of amylase released from the pancreas during a meal. This is a different genetic code than the amylase in saliva, but we won't get into that right now.
Amylase (Amy2B) genetic coding was found to have multiple copies in the domestic dog's genome compared to wolves.
So any pet food that markets their food as healthier because it doesn't contain fillers like corn or wheat is wrong, and are only using this as a marketing tactic.
Allergic to Grains
Some pet dogs have food allergies or food sensitivities. Allergens are protein peptide molecules that cause a hypersensitivity reaction in the body. Some of these hypersensitivities are Type I anaphylaxis - think about your acute facial swelling from a beesting, which others are Type II, III, or IV. Type IV is the more common reaction in our domestic dogs.
A Type IV allergy occurs less acutely - I.e. does not happen immediately. It is always why it makes it even more challenging to determine what the underlying cause is.
As a side note, most dogs with skin presentations of allergies have environmental allergies NOT food allergies. Food allergies are more common in cats than in dogs!
Anyway, it is the protein in the food item that causes an immune response through IgE where you get cytokine release (also why it is not responsive to antihistamines) and cell-mediated reaction in response to the cytokine release.
References
Axelsson, E., Ratnakumar, A., Arendt, M. L., Maqbool, K., Webster, M. T., Perloski, M., ... & Lindblad-Toh, K. (2013). The genomic signature of dog domestication reveals adaptation to a starch-rich diet. Nature, 495(7441), 360-364. [Link]
Ollivier, M., Tresset, A., Bastian, F., Lagoutte, L., Axelsson, E., Arendt, M. L., ... & Hänni, C. (2016). Amy2B copy number variation reveals starch diet adaptations in ancient European dogs. Royal Society Open Science, 3(11), 160449. [Link]
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