Most dog owners eventually face the challenge of properly feeding a geriatric dog. About one-third of all dogs in the United States are 11 or older, and dogs have an average life span of about 14 years, says Kathleen Hefner, DVM, a specialist in nutrition and nutritional counseling at the Animal Hospital of Saddle River in N.J.

But owners need to know when their dogs have reached senior status and require a dietary adjustment to meet their changing biological needs. Obvious signs include a graying snout and decreased mobility, as well as changes in activity level or interest in toys, games, or people. Owners should start looking for these signs from about age 7 – sooner for larger dogs and later for smaller ones.

Hefner emphasizes, however, that each dog is unique, so owners should consult their veterinarians and rule out any health problems before making dietary changes. All senior dogs are not the same, she says. You have to look at the more subtle things. You’ll want guidance on your individual pet’s needs.

But the majority of aging dogs face some common physical changes that require dietary changes to address them:

Dogs’ appetites can decrease as they age, so if they eat less, a calorie-dense diet can ensure they still get enough nutrients.

Dogs might also eat less because of pain from periodontal disease; more palatable, easier-to-chew food helps ensure Fido doesn’t go hungry.

Phosphorous and sodium can aggravate kidney problems, heart disease, and hypertension. If your dog has these illnesses, look for a diet with less of these two elements.

A sufficient amount of zinc helps keep the skin, coat, and immune system healthy.

Antioxidants, such as vitamins A, C, and E, and beta-carotene, are all believed to fight cancer and slow aging, so owners may want to supplement these if their dogs’ food doesn’t already include sufficient amounts.

Additives for joint health, such as glucosamine and chondroitin, often ease the aches of arthritis by maintaining the healthy cartilage that cushions the bones. Many senior dog foods include these ingredients, also available as supplements.

Aging dogs tend to have more gastrointestinal distress, so a diet with increased fiber can help prevent constipation.

Older dogs still require protein – vital to the body for cell repair and muscle maintenance – but can’t metabolize it as efficiently. They need higher-quality protein with a complete range of essential amino acids.

The omega-3 fatty acids found in fish oil and flaxseed oil can help alleviate a dull, dry coat and dry skin, as well as aid immune-system function.