I'm not an expert on this but I've heard alot about it. I heard it usually means they are lacking something in their diet - but I'm not sure what. Even though your dog has been on that food for some time, he is getting older and their bodies change and maybe at this age he's lacking something. At 7 or 8 (depending on the breed), they are already considered seniors. Hope this helps a little bit. Anyone out there know what it is that dogs lack when they do this?