loss of appetite in cats might be common, it should not be over looked. If your cat skips a meal now and then but otherwise eats properly, you should keep an eye out for anomalies. If your cat has stopped eating or is only eating a small quantity, you should contact your veterinarian. Even a few days without eating might result in fatty liver, also known as hepatic lipid sis.
Increased appetite could also be a serious concern, especially in old cats. Hypothyroidism could be the cause, but your veterinarian will be doing tests to be sure. Even in younger cats, increased appetite should not be underestimated. It can, at the at least, lead to overeating and obesity.