Vitamin B12
The vitamin vegans are told to watch. The body stores years of it, but only if it gets in.
Vitamin B12 is essential for red blood cell formation and nerve function, and it is found almost exclusively in animal foods, which is why plant-based eaters are routinely advised to supplement. Cyanocobalamin is the most common, stable, well-researched form, while methylcobalamin and hydroxocobalamin are active forms the body can use directly. There is no established upper limit because excess B12 is simply excreted, but very high-dose iron supplements can modestly reduce how much B12 gets absorbed if the two are taken together.
Reference values
Daily reference values from official nutrition authorities. These are population reference figures, not personal recommendations.
| RDA | Upper limit | Source |
|---|---|---|
| 1.5 mcg | No UL set | UK SACN DRVs (1991) 1.5mcg/day; EFSA NDA Panel — no numerical UL established for cobalamin (2015) |
| 1.5 mcg | No UL set | UK SACN DRVs (1991) 1.5mcg/day; EFSA NDA Panel — no numerical UL established for cobalamin (2015) |
Forms and bioavailability
Not all forms are absorbed equally. The form on your supplement label affects how much your body can actually use.
| Form | Absorption | With food | Timing | Evidence |
|---|---|---|---|---|
| Cyanocobalamin | Typical | With food | Morning | Strong |
| Methylcobalamin | Higher | With food | Morning | Likely |
| Hydroxocobalamin | Higher | With food | Morning | Likely |
How it pairs with other supplements
Works well with
B6 and B12 work together in homocysteine metabolism: B12 enables remethylation (methionine synthase pathway), B6 enables transsulfuration (cystathionine beta-synthase pathway). Combined supplementation shows additive homocysteine reduction.
Choline (via betaine), vitamin B12, and folate form the core methyl donor triad. Choline-derived betaine provides an alternative remethylation route when B12 availability is limited. B12 and choline demand is linked through shared one-carbon metabolism.
Folate and B12 are interdependent in the methylation cycle: 5-MTHF donates its methyl group to cobalamin for homocysteine remethylation. Deficiency of either impairs the pathway. High-dose folic acid can mask B12 deficiency by partially correcting megaloblastic anaemia without fixing neurological damage -- screen for B12 if supplementing folate at high doses.
Things to watch for
High-dose iron supplementation may modestly reduce vitamin B12 absorption via shared gastrointestinal uptake mechanisms. Space by 2+ hours if both are taken as standalone high-dose supplements.
Stack Almanac tracks which B12 form you're taking, adds up your intake across every source, and flags the iron pairing if you're taking a high-dose iron supplement at the same time.
Track your Vitamin B12 in Stack Almanac
Last reviewed: 2026-07-08. Reference values are adult-general RDAs from UK. Individual needs may vary. This page is for informational purposes only and is not a substitute for professional medical advice.