I don't know, but It's the way it works for some molecules. ie with niacin, your liver can't metabolize it totally, and the excess makes you flush badly. You can absorb only a fraction of it the 1st days (but if you take it on a daily basis, then all is metabolised inside cells, and you don't flush anymore)That's not how it works, the body absorbs what you give him.
Otherwise there would be no such thing as cocain overdose, as the body would reject the excess.