"Ash" in cat food is the inorganic mineral content left over when the organic portion has been removed. It generally consists of potassium, magnesium, and sodium salts, along with smaller amounts of other minerals. It used to be thought that the total "ash" content of food contributed to FUS, but recently, attention has focused on magnesium as the culprit. Many commercial foods now list the magnesium content as a separate item in the list of nutrients on the bag, box, or can."

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I hope it helps. Good luck!