Is it permissible for the head of nutrition and her colleagues to break their fast in the hospital kitchen with food items not designated for patients, given that the supplier is aware of this and it does not affect the patients' meals?

Since the food is designated for patients, the default ruling is that it is impermissible for others to eat from it, unless the hospital regulations explicitly permit staff to eat from it. Alternatively, if some food remains and would spoil or be thrown away, then it is permissible for them to eat from it at that point. The supplier's permission is not considered valid unless they are authorized by the hospital administration.