Vitamin B12 (Cobalamin)

•Observational— Standard nutritional biomarker

Why serum B12 alone misleads

Serum B12 (total cobalamin) includes inactive bound forms. ~5–10% of older adults with "normal" serum B12 have functional deficiency. Better indicators:

• Methylmalonic acid (MMA) — rises before serum B12 falls.
• Holotranscobalamin — the active fraction.
• Homocysteine — less specific but easier to measure.

If serum B12 is in the 200–400 zone and symptoms or risk factors are present, get MMA.

Clinical consequences of deficiency

• Megaloblastic anaemia (late).
• Peripheral neuropathy (often irreversible if prolonged).
• Subacute combined degeneration of the spinal cord.
• Cognitive impairment / dementia symptoms.
• Glossitis, mood disturbance.

Causes

• Inadequate intake (vegans, restricted diets).
• Pernicious anaemia (autoimmune loss of intrinsic factor).
• Atrophic gastritis (very common over age 60).
• Metformin (~20% of long-term users).
• PPIs (chronic use).
• Gastric or ileal resection / bypass.
• Crohn's disease.

Treatment

• Oral 1,000–2,000 μg/day is often sufficient even in pernicious anaemia (high-dose passive absorption).
• IM 1,000 μg weekly × 8 then monthly for severe deficiency or neurological involvement.
• Sublingual formulations may help in atrophic gastritis.

Related entries

Homocysteine, Methylmalonic acid, Metformin, Plant-based diet.