Another thing which makes bioavailability difficult is competition inhibition by other supplements, e.g. zinc and copper. There are also genetic changes between

Another thing which makes bioavailability difficult is competition inhibition by other supplements, e.g. zinc and copper. There are also genetic changes between individuals that make it hard to track; e.g. impaired folate metabolism due to MTHFR C677T gene. And then there’s the “does any of that really matter with respect to any metrics I care about” question, because presumably people are supplementing with some goal in mind beyond changing levels of minerals in their blood.