The 40 mg limit on zinc is really a limit on losing copper
The US limit sits at the point where zinc measurably suppresses a copper-carrying enzyme in red blood cells. Europe reads the same evidence and sets 25 mg a day.
What the study found
The 40 mg ceiling on zinc was set by what it does to copper
It is not a poisoning threshold. The US limit was chosen at the point where zinc measurably suppresses a copper-carrying enzyme in red blood cells, which makes the ceiling and the copper problem one story rather than two. Europe reads the same evidence and sets its limit at 25 mg a day.
The caveat · A long-term ceiling, not a harm line
An upper limit is a population ceiling for long-term intake from all sources, not the dose at which harm begins in a person. The short lozenge regimens that shortened colds ran at 80 to 92 mg of zinc a day and exceeded it deliberately, for about a week, which is not an error in either the trials or the limit. The US figure was set in 2001 and has not been revised for zinc since.
What we don't know
Europe caps zinc at 25 mg a day and the US at 40, from the same copper evidence. No new data separates them, only a judgement on how much copper loss is acceptable, and nothing settles which reading is right.