15/04/2026
B12 deficiency can mask folate deficiency. The relationship is more complex than originally thought.
Well worth a read.
B12 deficiency doesn't just lower your B12 levels. It traps your folate in a form your cells can't use. The result is functional folate deficiency despite adequate folate intake.
Here's why. Folate cycles through several forms inside your cells. One critical step is catalyzed by MTHFR, which converts 5,10-methyleneTHF into 5-methylTHF. This reaction is irreversible. Once folate enters the methyl form, there is only one way back: methionine synthase must strip the methyl group off, converting 5-methylTHF back to THF while simultaneously converting homocysteine to methionine.
Methionine synthase requires vitamin B12 as a cofactor. Specifically, it uses methylcobalamin. Without B12, the enzyme is inactive. The one exit from 5-methylTHF is sealed shut.
The consequence is that folate accumulates as 5-methylTHF, the one form it can't escape from, while THF, the form needed for DNA synthesis, is depleted. DNA replication depends on THF-derived thymidylate. Without it, rapidly dividing cells, particularly red blood cell precursors in the bone marrow, can't complete division. They enlarge without dividing. This is megaloblastic anemia.
Smulders et al. (2006, Br J Haematol) documented this trap directly in a B12-deficient patient. During deficiency, 94.5% of RBC folate was trapped as 5-methylTHF. After B12 supplementation, it dropped to 67.4%. Global DNA methylation was 22% lower during deficiency, and homocysteine was elevated at 52.9 compared to 16.8 after repletion. One important note: this patient was homozygous for the MTHFR C677T polymorphism, which reduces MTHFR activity and may have amplified the trapping effect. The paper describes this as "the first time that virtually all features of the methylfolate trap hypothesis have been demonstrated in a human."
Field et al. (2017, PNAS) confirmed the mechanism at the subcellular level. Using methionine synthase-null fibroblasts and nitrous oxide-treated cells, they showed that the nucleus is the most sensitive compartment to 5-methylTHF accumulation. Nuclear 5-methylTHF increased more than fourfold during B12 depletion. De novo thymidylate biosynthesis dropped 5 to 35%. DNA double-strand breaks increased.
This has two practical implications. First, B12 deficiency produces anemia that looks identical to folate deficiency on a blood smear. Both cause the same megaloblastic changes. The distinction requires measuring B12, methylmalonic acid, and homocysteine directly.
Second, and more concerning: high-dose folic acid can partially mask B12 deficiency by providing enough substrate to temporarily rescue DNA synthesis. The anemia improves. The blood smear normalizes. But folic acid does nothing for the neurological damage caused by B12 deficiency, which continues to progress silently. This is why B12 status should be assessed before or alongside folate supplementation, particularly in older adults.
Smulders et al., Br J Haematol, 2006 Field et al., PNAS, 2017
