good news, but the bad news is that those supplements can literally evaporate into the air. For that reason, unused food ought to be tightly sealed, preferably in an airtight container.
The plastic lids now commonly used to seal canned cat food work well (plus, putting an open can in the refrigerator slows the breakdown of vitamins). People do get careless with dry food, leaving the bag wide open or perhaps just rolling up the end. It’s better to fasten the end with a clothespin or, even better, dump the whole bag into an airtight plastic or glass container.