Bioavailability
The fraction of a swallowed dose that actually reaches your bloodstream in active form.
Bioavailability is what separates the dose printed on the bottle from the dose your body actually uses. A 1000 mg magnesium oxide tablet contains 1000 mg of compound but absorbs at roughly 4% — so the "delivered dose" is closer to 40 mg of elemental magnesium.
Two factors determine bioavailability:
- Form — chelated and citrate forms typically absorb better than oxide or carbonate salts.
- Co-ingestion — fat-soluble vitamins (A, D, E, K) absorb better with a meal containing fat. Iron absorbs better with vitamin C and worse with calcium or coffee.
When two products list the same number, the one with the better-absorbed form is the better deal even if the per-mg price is higher.