When you take an older dog or cat to the vet, over 6years, have the vet run a full blood panel. There is a disease called Cushing's disease. While this is usually not fatal to the animal it will cause conditions that will kill the animal. The symptoms you may observe are excessive drinking of water and a voratious appitite. If you are interested in knowing more you can got to:
http://www.canismajor.com/dog/cushings.html or Google on Cushing's disease.