Vitamin B12 deficiency operates through a counterintuitive metabolic mechanism that extends far beyond simple nutrient depletion. Recent research demonstrates that B12 deficiency traps folate inside cells in its 5-methylTHF form—a chemically inactive state that cells cannot utilize for DNA synthesis, even when dietary folate intake appears adequate.
The culprit is the enzyme methionine synthase, which requires methylcobalamin (B12) as a cofactor to convert trapped folate into usable forms. Without sufficient B12, this critical metabolic gate remains sealed. According to a landmark case study published in the British Journal of Haematology, 94.5% of red blood cell folate became metabolically inaccessible during B12 deficiency, yet normalized to 67.4% following B12 repletion.
This mechanism explains the paradoxical presentation of megaloblastic anemia and impaired cell division in B12-deficient patients despite apparently normal folate stores—a phenomenon clinicians have observed for decades but could not fully explain until now.
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