Many dog foods that are formulated for older dogs have lower levels of sodium, protein and phosphorous, but more fiber, to reduce the stress on the kidneys, heart and digestive tract; older dogs' digestive tracts absorb fewer nutrients, so vitamin supplements may be necessary. Consult with a veterinarian to determine the best diet to help your older dog gain weight.
If your dog is losing weight or shows other signs of health problems, you should request a complete veterinary examination to diagnose the cause, according to Medic Animal.
Older dogs often lose interest in food and may require more palatable flavors or textures. Dental problems also increase with age, which makes food harder to chew and swallow.
Other factors that may affect your older dog's overall health and appetite are his sleep, pain level and activity level. Pain from arthritis or other diseases may decrease his appetite. As your dog ages, he may become less active, which could adversely affect his mood and appetite. PetMD recommends finding new ways to play and interact with your aging dog to keep his spirits up.
Every breed and every individual dog is different. As your dog ages, make sure you take him in for twice-yearly checkups to adjust his diet and watch for other signs of disease.