When your pet ages, all its body tissues age. This includes the digestive system made up of stomach, intestines, and digestive glands like the pancreas and liver. Sudden diet changes can really disrupt an animals ability to digest their food and poor digestion can involve weight loss, vomiting, or diarrhea.
The dog that once was able to eat anything (and did) may become quite ill if it gets into the garbage or is fed scraps. Pancreatitis is a life threatening disease that can be induced by eating spoiled or excessively fatty food. In the dog, pancreatitis is recognized by symptoms and diagnostic tests like blood analysis and radiographs. In cats, pancreatitis is more difficult to diagnose and sometimes is impossible to confirm without an exploratory surgery or necropsy (an autopsy).
Sudden changes in an elderly pet's diet should be avoided. Use a gradual introduction of new foods. Even what would seem like a minor change, like a different flavor of kibble, may cause a problem, resulting in decreased appetite, vomiting, and/or diarrhea.
As an animal ages, it may actually be necessary to change the diet. Some animals benefit from special diets for diseases or conditions that are more common in the elderly, like kidney disease or liver disease or obesity. These diets changes should be discussed with your veterinarian. While salespeople in a pet food store have your pet's best interest at heart, ask about their educational background regarding nutrition and veterinary medicine. Be aware that what one animal may really do well eating, another may not be able to tolerate.
Obesity is often seen in older, sedentary pets, but as an animal becomes geriatric, it many times loses weight for various reasons. In cats, kidney and thyroid disease can cause significant weight loss. In both dogs and cats, cancer will many times result in weight loss. Any change in a pet's weight should be investigated by your veterinarian.
For some animals, the ability to digest and utilize all ingredients of a diet may be compromised by reduced enzyme production in the pancreas or liver. In some cases, it may become important to supplement commercial pet foods with home cooked foods like meat, and easy to digest carbohydrates like rice or potato. Additions of fatty acid supplements or vitamins may be necessary. Your veterinarian, or a veterinary nutritionist, can help determine if your pet would benefit from diet change or supplementation.