Vitamin bioavailability: why the form decides everything
The dose on the label says nothing about what your body absorbs. Bioavailability explained through four examples: B9, B12, magnesium, zinc.
You can take the right vitamin, at the right dose, and absorb almost none of it. The number on the label says how much is in the powder or the pill; it says nothing about how much reaches your blood. That difference has a name: bioavailability. It is decided, almost entirely, by the form of the nutrient, the exact molecule the manufacturer chose, written in small print in the ingredient list.
- Bioavailability = the share of a nutrient that actually reaches your bloodstream and your cells.
- Two products with the same dose on the front can differ several-fold in what you absorb.
- The form is never on the front of the pack. It is always in the ingredient list.
- Four examples decide most multivitamins: B9, B12, magnesium, zinc.
What bioavailability means
Bioavailability is the proportion of an ingested nutrient that crosses the intestinal wall and becomes available to your cells. It depends on the chemical form, on what you eat with it, on competing minerals in the same formula, and on your own physiology: age, stomach acidity, genetics. Of these factors, the form is the only one the manufacturer controls, and the only one you can read before buying.
Four examples that decide most formulas
Vitamin B9: folic acid vs 5-MTHF
Folic acid is synthetic and inactive as such: your body must convert it through several enzymatic steps. A large share of the population carries MTHFR gene variants that slow this conversion. The active form, 5-methyltetrahydrofolate (5-MTHF), skips the conversion entirely: it is the molecule your cells actually use. We covered this in detail in folic acid vs 5-MTHF.
Vitamin B12: cyanocobalamin vs methylcobalamin
Cyanocobalamin is the cheapest, most stable B12: your liver must remove a cyanide group and remethylate it before use. Methylcobalamin and adenosylcobalamin are the two forms your enzymes work with directly. After 50, when absorption declines for other reasons, the form matters even more: see B12 after 50.
Magnesium: oxide vs citrate and bisglycinate
Magnesium oxide packs the most elemental magnesium per gram, which is why it fills cheap tablets: but its measured absorption is among the lowest of all forms; a classic comparative study (Walker 2003) found citrate clearly superior. Citrate and bisglycinate are the forms with consistently good absorption data. Our full comparison: bisglycinate vs citrate.
Zinc: oxide vs citrate and bisglycinate
Same logic: zinc oxide is dense and cheap, with poor solubility in the gut. A randomised trial (Wegmüller 2014) measured zinc citrate absorption at the level of the reference gluconate, and clearly above oxide. Citrate and bisglycinate are the forms worth paying for.
Why cheap forms dominate the shelves
Oxides and synthetic vitamins cost less per kilo, are more stable in storage, and take less space in a tablet. None of these advantages concern your body. They concern the production line. This is not fraud: it is a choice of priorities, and the label lets you see it, if you know where to look.
How to read a label in 30 seconds
Ignore the front. Find the ingredient list and look for the words after each nutrient: oxide, carbonate, folic acid, cyanocobalamin on one side; citrate, bisglycinate, 5-MTHF, methylcobalamin, selenomethionine on the other. Then check the formula as a whole: even good forms compete with each other for absorption, which is why iron and zinc in the same dose is a red flag: we explain in vitamin antagonisms.
The choice made in base One
base One was formulated from this single criterion: every one of the 29 nutrients in its best-absorbed form, at a useful dose. Folate as 5-MTHF, magnesium and zinc as citrates, selenium as L-selenomethionine, biotin at 800 µg. And iron deliberately excluded, because it competes with zinc for the same transporter. The full logic is on the base One page.
FAQ
Is a higher dose better than a better form?
No. A poorly absorbed form at high dose mostly increases what leaves your body unused. A well-absorbed form at a sensible dose delivers more to your cells with less digestive load.
How do I know which form my supplement uses?
Read the ingredient list, not the nutrition table. The form is the word attached to each nutrient: magnesium oxide or magnesium citrate. If the list does not name the form, that silence is itself information.
Does bioavailability justify a higher price?
Active forms cost more per kilo, sometimes several times more. Whether that price is justified is visible on the label: you can check that the expensive product actually uses the forms it charges for.
Sources
Walker 2003 (Mg citrate vs oxide) · Wegmüller 2014 (zinc absorption by form) · Scaglione & Panzavolta 2014 (5-MTHF vs folic acid) · Obeid 2015 (cobalamin forms) · EFSA scientific opinions on nutrient sources.