Vitamin B12 & Folate
Vitamin B12 (Cobalamin) & Folate (Vitamin B9)
What it is
Vitamin B12 (cobalamin) and folate (vitamin B9) are water-soluble vitamins that function as essential cofactors in the one-carbon methylation cycle — the metabolic network responsible for DNA synthesis, DNA methylation, and the regeneration of methionine from homocysteine. B12 serves as a cofactor in two reactions: the remethylation of homocysteine to methionine (requiring 5-methyltetrahydrofolate donated by MTHFR activity) and the conversion of methylmalonyl-CoA to succinyl-CoA (in the mitochondria). Folate, in its active form 5-methyltetrahydrofolate (5-MTHF), is the methyl donor for homocysteine remethylation. Vitamin B12 is found almost exclusively in animal products — meat, fish, dairy, and eggs; deficiency is common in vegans, vegetarians, individuals with pernicious anaemia (autoimmune destruction of gastric intrinsic factor, which is required for B12 absorption), and those on long-term metformin or proton pump inhibitors (both of which impair B12 absorption). Folate is found in leafy green vegetables, legumes, and fortified foods; it is also universally recommended as a supplement in early pregnancy to prevent neural tube defects. Both are measured in serum; serum B12 can be supplemented by methylmalonic acid (MMA) and holotranscobalamin testing where functional B12 deficiency is suspected despite normal serum levels.
Why we measure it
B12 and folate deficiency both elevate homocysteine — which is independently associated with cardiovascular disease, stroke, and cognitive decline — but produce distinct patterns of clinical harm that make it essential to test both simultaneously rather than assuming they are interchangeable. B12 deficiency causes subacute combined degeneration of the spinal cord (demyelination of the dorsal and lateral columns, producing progressive proprioceptive loss, spastic weakness, and cognitive impairment), peripheral neuropathy, and megaloblastic anaemia. Folate deficiency causes megaloblastic anaemia and elevated homocysteine, but does not cause spinal cord degeneration. The critical clinical danger is this: high-dose folate supplementation can correct the megaloblastic anaemia of B12 deficiency — normalising the blood count — while the neurological damage from B12 deficiency continues unchecked and progresses to irreversible harm. This is why both must be measured together. In the context of MTHFR polymorphisms, both B12 and folate status directly determine whether homocysteine accumulates — a 677TT MTHFR individual with low B12 and low folate will have substantially elevated homocysteine regardless of MTHFR enzyme activity.
Why every 6 months
B12 and folate are measured biannually in the Pulse+ draw to provide sufficient resolution for tracking supplementation response and detecting deficiency before it produces clinical effects. B12 stores in the liver can sustain normal serum levels for several years despite zero intake, meaning deficiency develops slowly; but once neurological symptoms appear, some damage may be irreversible. The six-month interval ensures that the gradual depletion of B12 stores — particularly in individuals with absorption issues — is detected in time for intervention.
What movement means
Serum B12 and folate reference ranges vary by laboratory and assay. B12 is reported in pmol/L or pg/mL; folate in nmol/L or ng/mL. Standard reference ranges are derived from population distributions rather than functional thresholds — individuals can have neurological symptoms of B12 deficiency with serum B12 within the 'normal' range (particularly between 150–300 pmol/L), which is why methylmalonic acid (MMA) and homocysteine are useful functional indicators of actual B12 sufficiency.
B12 Deficient
< 148 pmol/L (< 200 pg/mL)
Below the standard deficiency threshold. Associated with megaloblastic anaemia and neurological damage. B12 supplementation or intramuscular injection (for absorption-related deficiency) is indicated. Intrinsic factor antibodies and gastric parietal cell antibodies should be checked to rule out pernicious anaemia.
NICE Vitamin B12 and Folate Deficiency Guideline, 2023 — cks.nice.org.uk/topics/anaemia-b12-folate-deficiency
B12 Borderline
148 – 300 pmol/L (200 – 406 pg/mL)
Borderline range where functional deficiency may be present despite technically 'normal' serum levels. Methylmalonic acid (MMA) and homocysteine should be measured to confirm functional adequacy. Particularly relevant in vegans, those on metformin, and anyone with GI absorption issues.
Stabler SP, New England Journal of Medicine, 2013 — doi:10.1056/NEJMcp1113996
B12 Sufficient
> 300 pmol/L (> 406 pg/mL)
Adequate B12 status. In individuals on B12 supplementation, levels well above this threshold confirm adequate repletion.
Standard laboratory reference ranges — vary by assay method
Folate Deficient
< 7 nmol/L (< 3 ng/mL)
Associated with elevated homocysteine and megaloblastic anaemia. In individuals with MTHFR 677TT genotype, supplementation with active 5-methyltetrahydrofolate (5-MTHF) is preferred over standard folic acid.
Standard laboratory reference ranges — vary by assay method
In the protocol
References
- 1.
Stabler SP. “Vitamin B12 Deficiency.” New England Journal of Medicine. 2013.
- 2.
Homocysteine Studies Collaboration. “Homocysteine and Risk of Ischemic Heart Disease and Stroke: A Meta-analysis.” JAMA. 2002.
- 3.
Smith AD, Smith SM, de Jager CA, et al.. “Homocysteine-Lowering by B Vitamins Slows the Rate of Accelerated Brain Atrophy in Mild Cognitive Impairment.” PLOS ONE. 2010.
- 4.
Carmel R. “Subclinical Cobalamin Deficiency.” Current Opinion in Gastroenterology. 2012.