B12 and Folate: One-Carbon Metabolism and the Order of Replacement

One-carbon metabolism: the shared job

To divide, a cell must build new DNA — and DNA needs single carbon atoms delivered on demand. Both vitamins serve one metabolic pathway: one-carbon metabolism, the shuttling of single carbon units used to make the building blocks of DNA. Folate is the actual carrier. In its active form, tetrahydrofolate (THF), it picks up a one-carbon fragment and donates it to build the purine bases and — critically — thymidine, the T of DNA. Without folate carriers, a dividing cell cannot make thymidine, cannot copy its genome, and stalls with its DNA half-built. The cells that divide fastest — the red-cell precursors in the bone marrow — feel this first, which is why the earliest sign is a failure of blood-cell production.

Where does B12 fit? It has two entirely separate roles, and the split is the key to everything. The first is the methionine synthase reaction: B12 (cobalamin) is the cofactor that transfers a methyl group from folate to homocysteine, making methionine and — as a by-product — regenerating active THF. Without B12 this step jams, and folate gets stuck in an unusable methylated form: the "methyl-folate trap." So a B12 deficiency causes a functional folate deficiency in the marrow, which is exactly why the two vitamins produce the same anaemia. The second B12 role is completely different: it is a cofactor for methylmalonyl-CoA mutase, a step in fat and odd-chain fatty-acid handling. This reaction does not need folate at all — and it is the one that keeps the nervous system healthy.

The same anaemia: megaloblastic red cells

Because both vitamins are needed to make DNA, a deficiency of either impairs nuclear maturation while the cytoplasm keeps growing. The result is a megaloblast: a red-cell precursor whose nucleus lags behind its swelling cytoplasm, giving abnormally large red cells (a high MCV) and hypersegmented neutrophils on the film. This is the classic megaloblastic anaemia. The blood picture is essentially indistinguishable between the two deficiencies — which is the whole problem. The fuller haematological detail — the blood-film findings, the reticulocyte response, the wider family of haematinics — is taught in the Haematology chapter on megaloblastic anaemia; here the emphasis is nutritional and pharmacological: what causes the shortfall, which drugs provoke it, and how to replace it safely.

Vitamin B12: an absorption story

B12 deficiency is almost never a diet problem in the well-fed — it is a plumbing problem. B12 comes only from animal foods (meat, fish, eggs, dairy), and the body stores years' worth in the liver, so dietary lack is slow to show. The vulnerability lies in a demanding absorption route. Dietary B12 must bind intrinsic factor, a protein made by the gastric parietal cells, and this complex is absorbed only in the terminal ileum. Anything that breaks either end of that chain causes deficiency. The classic cause is pernicious anaemia — an autoimmune destruction of the parietal cells and their intrinsic factor, so B12 can no longer be absorbed at all. Other causes follow the anatomy: gastrectomy or gastric atrophy (no intrinsic factor), and ileal disease, Crohn's, or ileal resection (no absorption site). A strict vegan diet, with no animal intake at all, is one of the few genuinely dietary causes. This absorption story links to the Gastrointestinal chapter — the stomach and terminal ileum are the two organs that matter.

And two drugs every exam loves: metformin and long-term PPIs. Two widely used drugs are recognised, testable causes of low B12. Metformin, the first-line drug for type 2 diabetes, reduces B12 absorption in the terminal ileum (a calcium-dependent effect), and long-term users can drift into deficiency — worth remembering, because a diabetic on metformin who develops a peripheral neuropathy may have low B12 rather than diabetic nerve damage. Long-term proton-pump inhibitors (omeprazole and its relatives) suppress gastric acid, and acid is needed to free B12 from food and let intrinsic factor bind it; years of acid suppression can lower B12. These drug causes tie into the Endocrine and Gastrointestinal chapters, where metformin and acid suppression are covered in depth. A separate, sinister mechanism belongs to the Toxicology chapter: nitrous oxide ("laughing gas," recreational or anaesthetic) irreversibly oxidises and inactivates the cobalt atom of B12, so heavy or repeated exposure can precipitate a full B12-deficiency syndrome — including the neurological one — even when stores were previously normal.

The neurology is what makes B12 dangerous to miss. Because of its second, folate-independent role, B12 deficiency does something folate deficiency never does: it damages the nervous system. The hallmark is subacute combined degeneration of the spinal cord — a progressive demyelination of the dorsal columns and corticospinal tracts, producing the classic picture of numbness and tingling, loss of vibration and position sense, an unsteady wide-based gait, and eventually spasticity. There may also be a peripheral neuropathy, and in severe or prolonged cases, cognitive change or dementia. The crucial and dangerous fact is that these neurological injuries can appear with only mild anaemia — or none at all — and if the deficiency is prolonged, they may not fully reverse even after treatment. That is why the neurological stakes, not the blood count, dictate the urgency and the order of replacement.

Folate: diet, demand, and drug antagonists

Folate has a very different profile. It comes largely from leafy green vegetables (folium is Latin for leaf), and body stores are small — only weeks to a few months — so deficiency appears far faster than with B12. Poor intake is a real cause, especially with alcohol excess, which impairs both intake and folate handling. Malabsorptive states such as coeliac disease reduce uptake. And any state of high cell turnover raises demand: pregnancy above all, but also haemolysis and malignancy. Folate deficiency also produces a megaloblastic anaemia, but — this is the whole point — no subacute combined degeneration. The nervous system is spared, because folate has nothing to do with the methylmalonyl-CoA step.

Several important drugs are folate antagonists by design or by accident. The most instructive causes of folate deficiency are pharmacological, because they reveal the pathway by blocking it. Methotrexate inhibits dihydrofolate reductase (DHFR), the enzyme that regenerates active THF — this is precisely how it kills dividing cells in cancer and calms the immune system in rheumatoid arthritis, and its predictable cost is a functional folate deficiency. Trimethoprim inhibits the bacterial version of the same enzyme, and in high doses or susceptible patients can nudge human folate metabolism too. Some antiepileptics, classically phenytoin, lower folate over time. These antifolate mechanisms connect directly to the Inflammation and Antimicrobials chapters, where methotrexate and trimethoprim are covered as therapeutic agents — the same enzyme block that treats disease is the one that can cause the anaemia.

Treatment and the public-health win

Replacing B12 depends on the cause. When absorption is the problem — pernicious anaemia, gastrectomy, ileal disease — the classic route is hydroxocobalamin by intramuscular injection, given as loading doses and then maintenance, bypassing the broken gut entirely. High-dose oral B12 can also work even in some malabsorption, because a small fraction is absorbed by passive diffusion independent of intrinsic factor. Folate is replaced with oral folic acid. When both are deficient, the order rule stands: correct or cover B12 first. A separate and important pharmacological form is folinic acid (leucovorin) — a ready-reduced folate that bypasses the DHFR block, which is why it is used as "folinic acid rescue" after high-dose methotrexate, restoring folate to healthy cells without reversing the drug's tumour effect, and as an antidote in methotrexate toxicity.

The most important use of folate is in a healthy person: preventing neural-tube defects. One of the great public-health interventions of modern medicine is preconception and early-pregnancy folic acid. The neural tube closes in the first weeks after conception — often before a woman knows she is pregnant — and adequate folate at that moment dramatically reduces the risk of neural-tube defects such as spina bifida and anencephaly. Standard advice is to start folic acid before conception and continue through the first trimester, with a much higher dose for higher-risk pregnancies — including women on folate-antagonist antiepileptics such as phenytoin, or with a previously affected pregnancy. This is why many countries fortify flour with folic acid at population level. The obstetric detail sits in the Obstetrics chapter; the point here is that a cheap water-soluble vitamin, given at the right moment, prevents devastating congenital disease.