Is there any research on how much of a supplement actually gets absorbed? very confusing when you see something like magnesium, daily limit of 420 mg. but then a magnesium bottle has a supplement pill of 500 mg that says to take 3 times a day. ?

Supplements=not drug: By definition supplements are not drugs and not subject to the pre-market safety or purity standards of the FDA. Their dosing suggestions are also not reviewed. The FDA only becomes involved if a supplement causes harm to persons & only if that harm is reported. There are independent labs like USP that does verify the product. You consume these at your own risk.