John is a 52-year-old factory worker. He has been admitted to the hospital and presents with muscle weakness and inflammation of his mouth. He is a bit confused and states that he isn't drinking now although he has a history of alcohol abuse. He has lost quite a bit of weight in the last six months, and his dietary intake has been poor. He was laid off a year ago but he is back to work part-time. Further testing has been ordered to determine the cause for concern and confirm the diagnosis. What food(s) would provide the most significant source of the vitamin missing in his diet