I'm not sure why they say these things are hard to understand as it seems like even an idiot could.
They're hard to understand because there isn't a direct one-one relationship between intake and serum levels, and different substances have complex interactions that can take years of experience to properly understand. As a simple example, if you're low on sodium - take salt, right? Well if you eat table salt or inject sodium chloride your sodium will go up, but so will your chloride, which causes acidosis if it gets too high. The purest form of dietary management is parenteral (intravenous) nutrition, which is what people get in ICU/ITU when they can't eat or take gastric feeds. It's incredibly complex and very easy to get wrong.