actually just copying the pertinent info on 'normal' here:
The first thing to understand when beginning nutrition investigations, is the unfortunate language of the lab. Specifically, the term 'normal'.
The 'normal' range is often quite a broad range which in many cases includes both sick and healthy people.
There is a much smaller range within the normal range that can be described as 'optimal'.
For example, the normal range for zinc is 10-20 umol/L. ms patients average in the low teens. healthy controls average in the high teens.
One example of the danger of the word 'normal' is this: recently a patient here at TIMS asked for a zinc test and was told the level was fine. When the patient asked for the number it came back 10.083 umol/L. This patient was not told that the level was suboptimal, or even borderline deficient, just that it was 'fine' - ie the computer did not red flag it because the value was within the min and max setpoints. (6/30 edit: i was reading a source today which listed the normal range for zinc as 11.5 - 18.5 umol/L so by that definition of normal, the TIMS patient was actually deficient. just a different set up at the lab and you're 'fine')
also my lab uses 8.7 as the bottom of the normal range, ever since my result came back that low. it used to be 11.5 before my deficient result got rolled into the lab's range. my stepdaughter had zinc tested at a lab used by an infectious disease clinic in vancouver and their normal range was 9-14 umol/L. my mum's lab uses the research numbers (so far) 11.5-18.5 umol/L.
according to research, optimal serum zinc is between 18 and 19 umol/L.