When considering the diet of your dog or cat, the food that will be the principal component of their daily diet, it is important to remember that they are carnivores. The majority of dog and cat foods available on the market contains a great deal of plant tissue in ratio to animal tissue. This would be fine if your dog or cat was an omnivore, but they are not. In the following post, I cite a few examples of their physiology as an argument for their carnivore status.
Omnivores (humans, pigs, bears) have the ability to effectively metabolize plant and animal tissue to meet their daily nutritional requirements. Carnivores (dogs, cats, whales) however, derive their nutrition and energy primarily and effectively from animal tissue rather than plant tissue. I consider dogs facultative carnivores, because they have the ability to digest and metabolize plant tissue, but they have no dietary requirement for minimum carbohydrate intake(1,2) for normal biological function. Plus, their digestive physiology has evolved to process diets high in animal tissue and low in plant tissue.
Dogs and cats have evolved physiological adaptions different from omnivores to consume and digest animal/plant tissue. Start with the mouth and jaw for example. The jaw of the dog is hinged to move in a vertical plane, not horizontal. Omnivores, such as you and me, have the ability to move the jaw vertically, horizontally, and front to back. We can chew in an up and down and grind or mash in a side to side motion. Very effective at increasing the surface area of plant tissue by mechanical means to aid in digestion. Dogs and cats do not have the ability to move their jaws in three different planes. Primarily, the jaw of a dog moves up and down with little side to side motion and no front to back motion. The carnivorous teeth structure of dogs is very different from an omnivore’s. They are more elongated (i.e. canines) to aid in grabbing prey and tearing. The carnivore’s molars are not as flat as an omnivore’s (useful for mashing plant matter) but more jagged to aid in shearing like scissors. And, because of muscle, joint, and teeth structure, the jaw closes like a pair of scissors from front to back, slicing through animal tissue.
Continuing with the mouth, the relative size of the oral cavity opening differs between carnivores and omnivores. Carnivores, generally have large mouth openings relative to head size to accommodate large pieces of food being ingested whole. An omnivore has a relatively small oral opening and narrower esophagus to accommodate smaller pieces of ingested and mashed up food pieces. The availability of digestive enzymes in the saliva are different, too. In omnivores, carbohydrates begin to break down in the mouth by an enzyme called amylase, which aids in the digestion of starch into monosaccharides. Dogs and cats do not possess amylase in their saliva, only pancreatic amylase is available for digestion of starches. Some familiar with dog nutrition claim this is an added stress to the pancreas when a large percentage of the diet is carbohydrate based. However, I was unable to find primary research results to indicate one way or another. But, one can surmise that the lack of salivary amylase is an indication that dogs and cats did not evolve to digest large dietary quantities of starches.
The digestive track of dogs and cats is relatively short in comparison to an omnivore’s. Animal tissue is easily digestible and absorbed readily in comparison to plant tissue, so the small intestine tract is short (5-6 times the body length compared to omnivores with tracts 7-11 times the body length). Animal tissue meals are not as readily available as plant meals; the carnivore has evolved a large stomach volume (relative to the entire digestive tract, usually around 60% of the total digestive tract volume) to gorge on a kill as an adaption to the lack of available food between meals. In comparison, the human stomach is only 21-27% of the volume of the entire digestive tract.
The pH of a carnivore’s stomach is also unique in comparison to an omnivore. Dogs and cats have a highly acidic pH of 1-2; suitable for digesting high quantities of protein and coping with bacteria associated with rancid meats. In contrast, the pH of our stomachs is 5-6.8 and can reach 3 during times of digestion. Carbohydrates are not broken down at a low pH (1-2), because enzymes like amylase are not functional at a pH below 6. So, in our systems ~50% of the breakdown of starches to monosaccharides occurs prior to stomach digestion and completed in the small intestine with pancreatic amylase. In a dog, carbohydrate digestion can’t begin until the plant tissue reaches the small intestine, pancreatic amylase is released, and the pH has increased. Couple that with the shorter small intestine tract and the evidence would indicate that dogs and cats are not physiologically adapted to digest large amounts of carbohydrates or plant tissue.
There is further physiological evidence, not mentioned here, which suggest dogs and cats have not evolved the physiological means to digest and metabolize a diet of predominately plant tissue. With that said, I am not suggesting that carbohydrates are bad for dogs and all carbohydrates should be removed from their diet. Their primary diets though, should not contain a large proportion of plant tissue to animal tissue especially of the starch variety, like grains. So when considering the best diet for your dog or cat, remember they are not physiologically like you or me, they are carnivores, and should be fed as such.
1. Waltham Book of Dog and Cat Nutrition (2nd edition, 1988)
2. Hematological and metabolic responses to training in racing sled dogs fed diets containing medium, low, or zero carbohydrates. Kronfeld, D.S., Hammel E.P., Ramberg, Jr. C.F., and Dunlap H.L. (1977) The American Journal of Clinical Nutrition (30)
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