Bioavailability
Also known as Absorption rate
The proportion of a nutrient or compound that actually reaches your bloodstream and can be used by the body.
Bioavailability describes how much of what you swallow is actually absorbed and put to use, rather than passed out unused. A supplement can contain a large dose on the label yet deliver little if its bioavailability is low.
Form, food, and timing all matter. Fat-soluble nutrients absorb better with a meal containing fat, some minerals compete with each other for uptake, and certain compounds are paired with absorption enhancers to raise the amount that gets through.