Trace Minerals

Needed in under 100 mg a day, several of them in micrograms. Trace minerals are mostly catalytic rather than structural: they sit in the active site of an enzyme or at the centre of a hormone, so a small shortfall can stop a specific reaction while everything around it looks normal.

That is also why the margin matters. Several of these — iron, copper, selenium, iodine — have a genuinely narrow window between too little and too much, and the symptoms at both ends can resemble each other.