The UrW nutrition model is a bit simplistic, so nutrients are neither gained nor lost by (poor) processing, but some (drying and smoking in particular) concentrate those nutrients in a smaller package, allowing characters to eat more nutrients before the belly gets full.
Water, naturally, doesn't contain any nutrients at all, and so is merely a filler. However, some foods need to be processed in various ways, and all kinds of processing are not available to all foods (you can't roast grain, for instance).