Protein is the foundation of Rex's nutrition, providing him with the 10 essential amino acids he can't live without. Though his protein needs will change as he ages, there's very little reason to feed him a low-protein diet. If he has too little protein, the effects will be apparent.
Don't Go Too Low
The amount of protein a dog needs depends on his age and some other factors. An adult dog's protein needs are different from a puppy's requirements; a lactating dog's nutritional requirements are not the same as those of a geriatric dog. A healthy adult dog requires that at least 18 percent of his daily caloric intake be protein. A puppy or lactating dog's diet should be at least 22 percent protein. Anything below those levels is a "low protein" diet.
Effects of Low Protein
The effects of a low protein diet depend on how low you go. If Rex's minimum requirements aren't being met, you'll see the impact. His coat won't be healthy -- his fur will be brittle or dry, perhaps even showing patches of hair loss. His skin may be darker in spots, or his fur may lose some pigmentation. As his body robs his protein to provide him energy for daily living, he'll feel the effects in other ways, perhaps losing weight and strength. Wounds may take longer to heal. A growing puppy, or an active or working dog, will have compromised bone and muscle development if his steady diet is low in protein.
About Those Kidneys
Traditionally it was believed a dog suffering from kidney failure should eat a low protein diet. The premise was that the already stressed kidneys wouldn't have to work harder to process the extra waste created by protein. However, that has been debunked; when your pup eats protein, it's digested and metabolized, releasing nitrogen that's excreted by the kidneys into the urine. Phosphorus is actually more difficult for the kidneys to process. Unless Rex has a blood urine nitrogen level above 75, you don't have to worry about cutting back his protein; in fact, he won't be harmed by a diet higher in protein.
Animal Proteins Are More Digestible
There's no agreement on how much is too much protein for a dog; but Dr. T. J. Dunn Jr. says dogs do fine eating a diet with protein levels of more than 30 percent on a dry weight basis. Dr. Dunn advocates focusing on animal protein, which is more digestible for your pup than plant-based protein, such as corn and soy. If Rex is getting on in years, you don't have to worry about feeding him a diet low in protein -- unless his BUN is above 75 or your vet recommends a different approach for something such as a liver disease.