What determines whether or not you only absorb a limited amount of a nutrient (like B12 or selenium) even after a heavy dose or overdose?

FOR B12 YOU ONLY ABSORB A LIMITED AMOUNT. What EVERYONE Should Be Doing About Methylation | Chris Masterjohn Lite #73 - YouTube

For b12 it’s intrinsic factor production which needs functional parietal cells that are stimulated to secrete gastric acid (eg when eating meat)