I have been told that dogs are carnivores because they inherited the trait from their ancestors, The Wolves. I also read an article online that says that dogs have been able to evolve over the years on meat and non-meat scraps and leftovers of humans. The article also says that 80% of dog food should be meat.
Are dogs actually carnivores or omnivores?
Is it true that 80% of dog food should be meat?
How will not feeding them meat affect their health?
Dogs are carnivores. They need meat to be healthy.
Dogs are carnivores, not omnivores. Dogs ARE very adaptable, but just because they can survive on an omnivorous diet does not mean it is the best diet for them. The assumption that dogs are natural omnivores remains to be proven, whereas the truth about dogs being natural carnivores is very well-supported by the evidence available to us.
That they can eat human left overs is an artifact of domestication in that it was a trait that would help an animal survive if it could eat left over food scraps it became more valuable. When dogs were first domesticated (prehistoric times) they were primarily kept around to take care of rats and other rodent pests.(source) Since table scraps would often attract these rodents a dog who would clean up the scraps would be more beneficial to the man. In addition this extra nutrition would help the dog survive during lean times.
How will not feeding them meat affect their health?
Dogs [do not] have the kinds of friendly bacteria that break down cellulose and starch for them. As a result, most of the nutrients contained in plant matter—even preprocessed plant matter—are unavailable to dogs. This is why dog food manufacturers have to add such high amounts of synthetic vitamins and minerals (the fact that cooking destroys all the vitamins and minerals and thus creates the need for supplementation aside) to their dog foods. If a dog can only digest 40-60% of its grain-based food, then it will only be receiving 40-60% (ideally!) of the vitamins and minerals it needs. To compensate for this, the manufacturer must add a higher concentration of vitamins and minerals than the dog actually needs. (source)
TL;DR version: There seems to be some argument about whether dogs are carnivores or omnivores. It is more important that you follow the guidelines for how much meat and plant matter to feed your dog than to worry about the taxonomic distinction that dogs fall into.
Humans and dogs (and pigs and bears) are monogastric omnivores. This simply means that they have one stomach [...] and consume a variety of animal and plant items. Pigs may eat far more vegetables and dogs much more meat, but we all find ourselves somewhere along the same continuum.
Cats, on the other hand, are obligate carnivores. While dogs can survive (though perhaps not thrive) on a vegetarian diet, cats must eat meat to fulfil their nutritional requirements. [Goes on to describe how dogs' teeth and taste buds are useful for eating vegetable matter.]
Source: Grab Life by the Leash
A dog's diet should consist of balanced porportions of proteins, carbohydrates, fats and, of course, water. A dog can go days without food and lose 30% to 40% of it's body weight without dying, but a 10% to 15% water loss could be fatal. All-meat diets are not recommended for dogs due to the lack of calcium and iron found in meat. Diet supplements should be avoided. Human foods that can be fatal to dogs include moldy cheese, onions, and chocolate. Feral domestic dogs will eat a variety of foods including animals and fruits.
Primary Diet: omnivore
Source: University of Michigan Museum of Zoology
Dogs do not normally produce the necessary enzymes in their saliva (amylase, for example) to start the break-down of carbohydrates and starches [...] Nor do dogs have the kinds of friendly bacteria that break down cellulose and starch for them. As a result, most of the nutrients contained in plant matter—even preprocessed plant matter—are unavailable to dogs.
Source (author doesn't really cite her experience - seems to be a personal website, albeit one with a lot of quotes)
With regards to keeping a dog as a pet, I'm not sure that the actual distinction matters. (If you want a deeper knowledge than that required to keep a dog as a pet, the Biology Stack Exchange should have plenty of experts who might be able to give a better answer than us.) Further, there is a large variation in how much meat is eaten within both the "carnivore" and "omnivore" spectrums.
The key things to note are:
How will not feeding them meat affect their health?
[T]he protein and calcium needs of the dog and cat are much higher than those for humans. These nutrients are most easily provided through animal-derived ingredients. Some plants, such as soy, are high in protein, but the amino acids within the protein are not as balanced as they are for most animal-source ingredients. Dogs and cats also need a dietary source of vitamin B12, a substance not found in most plants. All animals "need" this vitamin, but plant-eating animals such as cattle and sheep can make their own through the action of bacteria in their gastrointestinal tracts, provided there are adequate amounts of the mineral cobalt in the diet (which is found in plants).
How will not feeding them plants affect their health?
There's a distinction between different types of carnivores:
Animals that depend solely on animal flesh for their nutrient requirements are considered obligate carnivores while those that also consume non-animal food are considered facultative carnivores.
Source: Mammals: Carnivores. Duane E. Ullrey. Encyclopedia of Animal Science. - reference in Wikipedia
Even those sources that claim that dogs are carnivores are not claiming that dogs are obligate carnivores. In other words, even if they are carnivores, they are not the type of carnivore that eats only meat.
Further:
[T]here is no such a thing as an all-meat diet in the wild. Wild wolves inevitably digest a fair amount of plant matter too – as they benefit from the plant diet of the prey animals they hunt.
And finally, as quoted above:
All-meat diets are not recommended for dogs due to the lack of calcium and iron found in meat.
Dogs are carnivores but you need to keep in mind that when dogs eat an animal (mice or squirrels for instance), they eat the whole animal and that includes partially digested plants and grains in the stomach and digestive system. This can be substituted with meat (muscle), bone and plants, as is done in commercial dog food.
