Vitamin B12
Essential for energy production and neurological function
What is Vitamin B12?
Vitamin B12 (cobalamin) is a water-soluble vitamin found almost exclusively in animal products. It's essential for DNA synthesis, red blood cell formation, myelin sheath maintenance (the insulating layer around nerve fibres), and energy metabolism via the methionine cycle. B12 deficiency causes megaloblastic anaemia and can cause irreversible neurological damage if untreated. In the UK, B12 deficiency affects approximately 6% of adults under 60 and rises steeply with age.
How it works
B12 functions as a cofactor for two critical enzymes: methionine synthase (which converts homocysteine to methionine, required for DNA methylation) and methylmalonyl-CoA mutase (required for odd-chain fatty acid metabolism and succinyl-CoA production in the Krebs cycle). Deficiency leads to folate trapping (functional folate deficiency), elevated homocysteine (cardiovascular risk factor), impaired myelin synthesis, and megaloblastic anaemia.
What the evidence shows
B12 deficiency is well-established as a cause of fatigue, depression, cognitive decline, peripheral neuropathy, and megaloblastic anaemia. Supplementation reverses deficiency reliably. A 2016 Cochrane review found B12 supplementation significantly reduced depression symptoms in deficient patients. Elevated homocysteine (driven by B12 deficiency) is an independent cardiovascular risk factor — supplementation consistently lowers it.
When to expect results
Red blood cell production normalising if anaemia was present. Energy beginning to improve.
Fatigue lifting, mood stabilising. Neurological symptoms (tingling, weakness) beginning to resolve if early-stage.
Homocysteine levels normalising. Cognitive clarity improving. Blood counts returning to normal.
Full neurological recovery (if nerve damage was not severe). Sustained energy, mood, and cognitive function.
Dosing
For deficiency prevention: 10–25mcg daily is sufficient. For correction of deficiency: 1,000mcg (1mg) daily is recommended by NICE for those with confirmed deficiency and no absorption issues. Vegans and vegetarians should supplement consistently. Note that B12 absorption from supplements is largely passive (does not require intrinsic factor) — so even people with absorption problems can benefit from high-dose oral supplementation.
Forms to choose
Methylcobalamin (most active form, best for neurological protection), adenosylcobalamin (mitochondrial form, used alongside methylcobalamin in some products), and cyanocobalamin (synthetic, most stable, converts to active forms in the body — effective in trials). Hydroxocobalamin is used in injections. For supplements: methylcobalamin or a combination is optimal.
Who benefits most
Vegans and vegetarians (essential — no reliable plant sources), older adults (intrinsic factor production declines with age), people on metformin (impairs B12 absorption), those on PPIs or H2 blockers (reduce stomach acid needed for B12 release from food), anyone with megaloblastic anaemia or neurological symptoms.
Who should avoid / caution
No significant contraindications. Leber's disease (hereditary optic neuropathy) is the one rare case where cyanocobalamin form should be avoided (use methylcobalamin instead).
Interactions & stacking
B12 and folate work together in the methionine cycle and DNA synthesis. Deficiency in one creates functional deficiency in the other.
Metformin impairs B12 absorption. People on metformin should supplement B12 and monitor levels annually.
Acid-suppressing medications impair B12 release from food (requires stomach acid). Long-term users should supplement.
B6, B12, and folate together reduce homocysteine most effectively — a triple combination often used for cardiovascular risk reduction.
Safety & side effects
Vitamin B12 is extremely safe — it's water-soluble and excess is excreted. No established upper limit or toxicity threshold. Injections occasionally cause reactions at the site. Very rarely, high-dose supplementation can trigger or unmask acne in predisposed individuals.
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This information is for educational purposes only and does not constitute medical advice. Always consult your GP or a qualified healthcare professional before starting any new supplement, particularly if you are pregnant, breastfeeding, have a medical condition, or are taking prescription medication.