What is bioavailability?
Bioavailability is how much of something you swallow actually gets absorbed and reaches your bloodstream in a usable form.
Imagine mailing 100 letters, but only 10 arrive at the right address. The other 90 got lost or destroyed along the way. That 10 percent is the bioavailability.
Why it matters for aging
After you swallow a supplement, it has to survive stomach acid, get absorbed through the gut wall and pass through the liver, which breaks down many substances before they reach the rest of the body. Each step can lose a big share.
That's why the form matters. Some magnesium forms absorb far better than others, and NAD⁺ taken as a pill is mostly broken down before it reaches cells. When a supplement works in a lab dish but not in people, poor bioavailability is often the reason.
A common myth
If a supplement contains a big dose, your body gets a big dose.
Many compounds are poorly absorbed or rapidly broken down by the gut and liver. Curcumin from turmeric and resveratrol are famous examples: large doses go in, very little reaches the blood intact.
Go deeper
Deep dives
- Which type of magnesium for what?
- Resveratrol: what happened to the red wine miracle?
- NMN vs NR vs NAD⁺: what actually raises NAD⁺, and does it matter?
Related terms
Updated Nov 9, 2026 · All terms