05/09/2026
Every nerve in your body is wrapped in myelin, a multilayered lipid sheath that insulates the axon and lets electrical signals travel fast. B12 maintains this sheath through two separate pathways, and both fail at the same time when B12 drops.
The first pathway runs through methionine synthase. B12 (as methylcobalamin) converts homocysteine into methionine, which becomes SAMe. SAMe provides the methyl groups needed to build phosphatidylcholine, one of myelin's major lipid components. When B12 is low, SAMe drops and myelin lipid synthesis stalls.
The second pathway runs through methylmalonyl-CoA mutase. B12 (as adenosylcobalamin) clears methylmalonyl-CoA by converting it to succinyl-CoA. Without B12, methylmalonic acid (MMA) accumulates and abnormal fatty acids get incorporated into the myelin sheath itself, weakening its structure from the inside.
What makes this dangerous: the nerve damage can appear before anemia does. In the Lindenbaum NEJM study, 28% of patients with neuropsychiatric symptoms from B12 deficiency had completely normal blood counts. No anemia, no macrocytosis. The myelin was already degrading with nothing on the CBC to flag it. If caught early, repletion can reverse it. If caught late, the demyelination can become permanent.
Scalabrino, Prog Neurobiol, 2009.
Lindenbaum et al., NEJM, 1988.
