Serum mineral levels are not super useful because many minerals are stored in bones and tissue.

Before they are stored there, they are taken there via blood

If I want to take stock of my available goods, do I count how much cargo vans going in and out of the warehouse are carrying, or do I have to go inside and take a look at what’s actually in store?

Tap water, like blood, is a flow not a stock

We don’t need minerals for blood (hardly any consumers there), but for tissue and structure. Those sites store and don’t flow.