Définition
Methylcobalamin is the active, coenzyme form of vitamin B12 — the form your body uses directly for methylation and nervous system function. It is preferable to the synthetic cyanocobalamin form found in most cheap B-complex supplements, particularly for methylation-sensitive individuals.
Vitamin B12 exists in several forms. Cyanocobalamin is a synthetic, stable form used in most fortified foods and generic supplements — but it requires the body to remove a cyanide group and convert it to an active form before use. Methylcobalamin is one of the two active forms (adenosylcobalamin is the other), and can be used directly by cells without conversion. Hydroxocobalamin is an intermediate form, often used in injections.
B12 has two primary biological roles. First, it is a co-factor for methionine synthase, the enzyme that converts homocysteine back to methionine in the methylation cycle. B12 deficiency causes homocysteine to accumulate and methylation capacity to decline. Second, it is required for nervous system myelination — B12 deficiency produces peripheral neuropathy, cognitive symptoms, and in severe cases spinal cord degeneration.
B12 deficiency is surprisingly common, particularly in: plant-based eaters (B12 is essentially absent from plant foods), adults over 50 (declining stomach acid impairs B12 absorption from food), women taking hormonal contraceptives (which can reduce B12 status), and individuals with impaired gut function. Serum B12 testing misses many functional deficiencies — methylmalonic acid (MMA) and homocysteine are more sensitive functional markers.
Methylcobalamin supplementation at 500–1000 mcg daily (sublingual or oral) is effective for most deficiencies. Sublingual administration is particularly useful for older adults with absorption issues. For women with elevated homocysteine despite adequate folate, adding methylcobalamin alongside methylfolate is typically more effective than either alone — they work together in the same enzymatic reaction.
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