Bioavailability

Bioavailability is how much of what you swallow reaches the circulation in an active form. It depends on the chemical form of the compound, what it is eaten with, and the state of your gut — the same nutrient can differ several-fold across forms.

It matters because the amount on a label is not the amount you get. Haem iron from meat is far better absorbed than non-haem iron from plants; curcumin is poorly absorbed on its own; magnesium oxide is much less bioavailable than several organic salts. It is one of the main reasons two supplements with identical stated amounts are not equivalent.

In practice it also cuts the other way: some compounds are marketed on an 'enhanced absorption' claim that has been tested only against a deliberately poor comparator. It is a real variable, and also a favourite of supplement marketing.