We have a very thin Irish Setter. Have had him a couple years, always had good food available (Diamond at the feed store, check the ingredients), stayed thin. Recently we put him on Rimadyl because he's old and slowing down. He started eating much more. I am guessing he had/has some arthritis in his jaw from an injury and just couldn't eat enough without hurting. First time he ate a whole bowl of food in one day was the first day he took a pain med.