A proper diet helps ensure that your fury friend stays away from that rainbow bridge for as long as possible.

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Anybody who has ever loved a senior dog knows that beneath every graying snout lies the soul of a puppy. But time doesn't wait for anyone, and even the most rambunctious puppies get older.

As Mark Nunez, DVM, president of the California Veterinary Medical Association, explained to WebMD, when a dog is considered "old" depends on their breed. "Little dogs live to about 15 to 20 years of age, while bigger dogs live to about 12 to 15 years," Nunez said. "Bigger dogs are considered older at around six years, and smaller dogs become older at around eight or nine."

And like the humans they love so much, a dog's nutritional requirements change as they age.

"Seniors and geriatrics generally need lower-calorie diets to help prevent obesity—a huge problem in seniors—and higher-fiber diets to improve gastrointestinal health," Metzger told WebMD.

"Probably the most important thing for a geriatric dog is that their energy requirement gets lower," Metzger continued. "With a slower metabolic rate, older dogs are more likely to become overweight or obese."

If possible, feed your dog food that is specific to his stage in life. Many dog food companies now offer senior dog food formulations, which are good for older canines because they're lower in calories. The foods usually have added joint supplements and omega 3 oils and fatty acids to help their joints.

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When it comes to nutrition, foods for senior dogs need to be protein-rich to maintain muscle mass. But, as PetSide points out, not too much protein. Keep in mind that as protein content rises, so does the phosphorus content in foods which could be a concern for senior pups with kidney problems. Fiber is also an important component to get your dog pooping, well, regularly well into his twilight years.

To make sure your old dog gets enough antioxidants (which are great for maintaining brain health and preventing cancer!) consider supplementing his food with fresh veggies like carrots and sweet potatoes.

And most importantly, make sure your senior pet gets plenty of love.