Vitamin B12 is synthesised exclusively by bacteria and archaea. Animals obtain it from these microorganisms through their digestive systems or through soil bacteria consumed inadvertently. Humans consuming animal products obtain it from the accumulated B12 in those tissues. Whole plant foods, while sometimes containing B12-producing bacteria on their surfaces before washing, do not provide reliable B12 for humans. Fermented plant foods and algae contain B12 analogues that are poorly absorbed and may compete with genuine B12 at absorption sites.
The Consequences of Deficiency
B12 deficiency progresses slowly because the liver stores 2 to 5 years of B12. Early stages are detectable through elevated homocysteine and methylmalonic acid before neurological symptoms appear. Established deficiency causes irreversible neurological damage including peripheral neuropathy, subacute combined degeneration of the spinal cord, and megaloblastic anaemia. The insidious nature of deficiency means that people often do not notice symptoms until significant neurological damage has occurred.
The Correct Supplementation Approach
- Cyanocobalamin is the most stable and best-researched form for supplementation; the body converts it to the active forms methylcobalamin and adenosylcobalamin
- High-dose weekly supplementation (2,000 micrograms weekly of cyanocobalamin) is as effective as daily low-dose supplementation and may suit some people better
- Daily supplementation at 25 to 100 micrograms provides adequate amounts for most adults on wholly plant-based diets
- Regular blood testing (B12, homocysteine, MMA) is prudent to confirm adequacy of supplementation, as absorption varies between individuals