As our pets age, their dietary needs may also change. They may become less active, and require fewer calories than they once did. They may be more prone to constipation, and require more fiber. If they have decreased kidney function, they may benefit from a diet lower in phosphorus.
Diets should maintain a healthy skin, coat and body weight. Older pets may benefit from additional Omega-6 fatty acids, vitamin E and beta-carotene antioxidants to help reduce or slow diseases.
To maintain muscle mass, make sure your pets are receiving high quality proteins. Egg, meat, and fish are highly digestible and will give your pet the most usable amino acids. Lower-quality foods will have inexpensive ingredients (like corn) that aren't readily absorbed by your pet's digestive system. Nutrients are more likely to pass through, so they would need to eat more of the low quality food to get the same nutrition as a high-quality food.
Depending on your ill or elderly dog's situation, they may require more protein, or far less. Always consult with your veterinarian, and discuss your pet's lifestyle, any changes in eating or drinking habits, and possible diseases to consider.