Nature made the rules of biochemistry and nutrition and we mortals have no power (and no business, for that matter) to try to bend those rules.
For that reason there are truly no adequate vegetarian diets for cats.
Overweight dogs, dogs with itchy, flaky skin, dogs with coarse and brittle coats, dogs with poor energy levels and resistance to infection -- 95 percent of the time these dogs will be consuming diets low in animal origin tissues and high in grain-based products. Remember, though, that grains provide mostly carbohydrates and only limited amino acid (protein) profiles. The very early research that pointed a finger at protein as being a cause of kidney failure in dogs wasn't even done on dogs!
Inexpensive, corn-based diets are some of the worst. Extra carbohydrate intake, above the immediate needs of the dog (which occurs often with grain-based diets) prompts internal factors to store that extra carbohydrate (sugar) as fat. It was done on rats fed unnatural diets for a rodent -- diets high in protein.
We all need to have an open mind and take a look at what by-products really are.) "But too much protein is bad, right? Do your own research and poll half a dozen nutrition specialists (not the guy who runs the local pet shop) and here is what you will find: There is no general agreement among expert nutritionists regarding what constitutes “too much” protein in the dog’s diet.
Research shows that dogs have a high capacity for digesting and utilizing diets containing more than thirty percent protein on a dry weight basis.
Nature has created a meat-eating machine in the dog and every day in practice I see the health benefits displayed by the feeding of meat-based diets. "I’ve always been told that high protein diets are bad for an older dog's kidneys; even my veterinarian says so." What researchers have proven is this: In dogs that actually have kidney damage or dysfunction (regardless of their age) and that have a BUN level greater than 75, restricted protein intake may be beneficial but not because of any adverse impact on the kidneys.
The other 12 required amino acids can be manufactured internally in the dog’s liver.
They can survive on a diet of either plant or animal origin if it is balanced and diverse.
But to thrive and not merely survive, dogs should have a source of animal protein -- MEAT! There is a huge difference between survive and thrive!
These authors point out that phosphorus blood levels can play a major role in the health status of dogs with compromised kidney function.
Here's another expert opinion: "The dog can digest large amounts of proteins, especially those of animal origin" stated Prof. D., at the Fourth Annual International Sled Dog Veterinary Medical Association Symposium (page 53 of 1997 PROCEEDINGS).