Vitamin B12 and fatigue: separating fact from fiction

Every B12 deficiency causes fatigue. But not every fatigue is a B12 deficiency. How to sort it cleanly, without falling into self-diagnosis.

Vitamin B12 is a coenzyme essential to DNA synthesis and red blood cell formation. Its deficiency produces a macrocytic anaemia with marked fatigue, peripheral neuropathies and sometimes cognitive trouble. But the reverse, that all fatigue stems from B12, is false.

A B12 deficiency does cause marked fatigue, but not all fatigue is a B12 deficiency. At-risk profiles are vegans, the over-65s, patients on metformin and bariatric-surgery patients. Diagnosis isn't limited to serum B12 (add holotranscobalamin, homocysteine). The preferred form is methylcobalamin.

Who is really at risk?

• Strict vegan adults without supplementation (very high risk).
• People >65 (gastric parietal atrophy reduces absorption).
• Patients on long-term metformin (proven interference).
• History of bariatric surgery (gastric resection).
• Pernicious anaemia or gastric autoimmunity.

Diagnosis: not just serum B12

Measuring blood B12 gives an incomplete picture. For a robust diagnosis, the doctor may add: holotranscobalamin (active form), plasma homocysteine, urinary methylmalonic acid.

Which form to prefer?

Methylcobalamin sublingual or in a capsule. It's the active form, directly usable. Cyanocobalamin remains functional but requires conversion. Tissue retention is superior with methylcobalamin (Paul & Brady, 2017).

What dosage?

Adult without deficiency: 2.5 µg/day (NRV) is enough. That's the dose in the Chutex Complete Multivitamin. Adult with an identified deficiency: 1000 µg/day for 4 weeks then 1000 µg/week as maintenance (on medical advice).

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