B12 vs folate deficiency: MMA, homocysteine and the nerves
How to tell B12 deficiency from folate deficiency using methylmalonic acid, homocysteine and the nervous system, and why folic acid alone can fix the blood while the nerves worsen.
Chapters (14)
Distinguish vitamin B12 deficiency from folate deficiency using methylmalonic acid, homocysteine and neurological findings, based on their roles in one-carbon metabolism.
The short version
- Both deficiencies stall DNA synthesis and give the same megaloblastic anaemia, with large red cells and hypersegmented neutrophils.
- Homocysteine is high in both, because methionine synthase needs folate and vitamin B12.
- Methylmalonic acid rises only in B12 deficiency, because methylmalonyl-CoA mutase needs B12 and has no folate role.
- Peripheral neuropathy and subacute combined degeneration are B12 complications that can appear without anaemia; subacute combined degeneration is not a folate complication.
- In megaloblastic anaemia, exclude B12 deficiency before giving folate alone.
The case: folic acid fixes the blood, not the nerves
A 66-year-old woman has a macrocytic anaemia with an MCV of 118 fL. She is started on folic acid, and within weeks her blood count improves. Over the next months, though, her feet go numb and her walking becomes unsteady. What went wrong?
The answer sits in the difference between B12 vs folate deficiency. This lesson separates the two using methylmalonic acid (MMA), homocysteine and the nervous system.
Why both vitamins matter: DNA synthesis and two enzymes
Folate and vitamin B12 both feed DNA synthesis, so both are needed to make new blood cells.[1][3] Folate carries one-carbon units, which are used to build thymidylate and purines, the raw materials of DNA.[4][5]
The folate cycle and the methionine cycle meet at one enzyme, methionine synthase. It needs vitamin B12 to move a methyl group from methyl-tetrahydrofolate onto homocysteine, making methionine.[4][5]
Vitamin B12 has a second job, in mitochondria: methylmalonyl-CoA mutase turns methylmalonyl-CoA into succinyl-CoA. Folate has no part in that second reaction.[5][6]
| Enzyme | Reaction | Needs folate? | Needs vitamin B12? |
|---|---|---|---|
| Methionine synthase | Methyl group from methyl-tetrahydrofolate onto homocysteine, making methionine | Yes (methyl donor) | Yes (cofactor) |
| Methylmalonyl-CoA mutase | Methylmalonyl-CoA → succinyl-CoA, in mitochondria | No | Yes |
Megaloblastic anaemia: the blood picture is identical
In folate deficiency, without one-carbon units, DNA synthesis stalls. Marrow cells grow but divide late, giving a megaloblastic anaemia, with large red cells and neutrophils that have too many lobes.[3][6]
Vitamin B12 deficiency stalls the same folate cycle, so the blood picture is identical.[5][6] The blood count and smear cannot separate the two; the biochemistry and the nervous system can.
Methylmalonic acid and homocysteine: the key test
Follow each block. In folate deficiency, homocysteine builds up because methionine synthase has no methyl donor, while methylmalonic acid stays normal.[2][3]
In B12 deficiency, the methionine synthase block raises homocysteine as well. But the mutase block belongs to B12 alone: methylmalonyl-CoA backs up and methylmalonic acid rises.[2][5][6]
| Folate deficiency | B12 deficiency | |
|---|---|---|
| Megaloblastic anaemia | Yes | Yes |
| Hypersegmented neutrophils | Yes | Yes |
| Homocysteine | High | High |
| Methylmalonic acid | Normal | High |
| Nerve damage (subacute combined degeneration) | Not a feature | Yes |
In US exam teaching, a serum B12 below 200 pg/mL indicates deficiency[2]; methylmalonic acid helps when the level is borderline.[2][5]
The nervous system: subacute combined degeneration
The nervous system is the second clue. B12 deficiency causes peripheral neuropathy and subacute combined degeneration of the spinal cord.[2][7]
- Dorsal columns: vibration and position sense are lost.
- Corticospinal tracts: weakness and spasticity.
The nerve damage can appear without anaemia, and subacute combined degeneration is a B12 complication, not a folate one.[2][6][7]
Time and causes: how fast each deficiency develops
Time is a clue too. If intake stops, liver stores of B12 last three to five years, but folate stores last only a few months.[2][3][9] So B12 deficiency usually develops slowly, while folate deficiency can come fast.
| B12 deficiency | Folate deficiency | |
|---|---|---|
| Stores if intake stops | 3–5 years | A few months |
| Causes | Pernicious anaemia (autoimmune gastritis destroys the parietal cells that make intrinsic factor); loss of the terminal ileum; a strict vegan diet; metformin | Poor diet; alcohol; pregnancy; antifolate drugs such as methotrexate |
Pernicious anaemia, loss of the terminal ileum, a strict vegan diet and metformin are B12 causes[2][5][8]; poor diet, alcohol, pregnancy and methotrexate are folate causes.[3][9]
Why the patient got worse: exclude B12 before giving folate
That is why the patient in the opening case got worse. Folic acid can correct the anaemia while the nerve damage progresses.[2][6]
Practice question
Original · USMLE styleA 62-year-old man has had 4 months of numbness in his feet and an unsteady gait. Vibration and position sense are reduced in both legs. Haemoglobin is 9.4 g/dL and the mean corpuscular volume (MCV) is 112 fL. Serum folate is normal.
Common questions
How do you tell B12 deficiency from folate deficiency?
Use methylmalonic acid and the nervous system. Both deficiencies give the same megaloblastic anaemia and raise homocysteine, but methylmalonic acid rises only in B12 deficiency, which also damages the nerves.
Why does folic acid not treat B12 deficiency fully?
Folic acid can correct the anaemia of B12 deficiency while the neurological damage progresses. In megaloblastic anaemia, B12 deficiency should be excluded before folate is given alone.
Why is homocysteine high in both deficiencies?
Methionine synthase needs folate as the methyl donor and vitamin B12 as the cofactor, so a shortage of either one makes homocysteine build up.
What is subacute combined degeneration?
It is a B12 complication of the spinal cord affecting the dorsal columns (loss of vibration and position sense) and the corticospinal tracts (weakness and spasticity). It can appear without anaemia.
What serum B12 level indicates deficiency?
In US practice, a serum B12 below 200 pg/mL indicates deficiency; methylmalonic acid helps when the level is borderline.
Transcript
Read the full narration, by chapter611 words
0:00The case: folic acid fixes the blood, not the nerves, and what to separate today
A 66-year-old woman has a macrocytic anaemia, with an MCV of 118 femtolitres. Her doctor starts folic acid, and within weeks her blood count improves. But over the next months her feet go numb and her walking becomes unsteady. What went wrong? Today: tell vitamin B12 deficiency from folate deficiency using methylmalonic acid, homocysteine and the nervous system.
0:28Both vitamins feed DNA synthesis
Both vitamins feed DNA synthesis, so both are needed to make new blood cells. Folate carries one-carbon units, used to build thymidylate and purines, the raw materials of DNA.
0:41The folate cycle meets the methionine cycle at methionine synthase
The folate cycle and the methionine cycle meet at one enzyme, methionine synthase. It needs vitamin B12 to move a methyl group from methyl-tetrahydrofolate onto homocysteine, making methionine.
0:55B12's second job: methylmalonyl-CoA mutase
Vitamin B12 has a second job, in mitochondria: methylmalonyl-CoA mutase turns methylmalonyl-CoA into succinyl-CoA. Folate has no part in that second reaction.
1:09Folate deficiency: megaloblastic anaemia and hypersegmented neutrophils
Folate deficiency first: without one-carbon units, DNA synthesis stalls. Marrow cells grow but divide late, giving a megaloblastic anaemia, with large red cells and neutrophils that have too many lobes.
1:24Homocysteine up, methylmalonic acid normal
Homocysteine builds up, because methionine synthase has no methyl donor. So in folate deficiency, homocysteine is high and methylmalonic acid is normal.
1:36B12 deficiency: the same blood picture, two blocks
Vitamin B12 deficiency stalls the same folate cycle, so the blood picture is identical. The methionine synthase block raises homocysteine as well. But the mutase block belongs to B12 alone: methylmalonyl-CoA backs up and methylmalonic acid rises. So in B12 deficiency, both homocysteine and methylmalonic acid are high.
2:01Serum B12 below 200 pg/mL, and when MMA helps
In US practice, a serum B12 below 200 picograms per millilitre indicates deficiency; methylmalonic acid helps when the level is borderline.
2:12The nervous system: subacute combined degeneration
The nervous system is the second clue. B12 deficiency causes peripheral neuropathy and subacute combined degeneration of the spinal cord. That means the dorsal columns, so vibration and position sense are lost, and the corticospinal tracts, so there is weakness and spasticity. The nerve damage can appear without anaemia, and subacute combined degeneration is a B12 complication, not a folate one.
2:40Time clue: B12 stores last years, folate months
Time is a clue too. If intake stops, liver stores of B12 last three to five years, but folate stores last only a few months. So B12 deficiency usually develops slowly.
2:55Causes of B12 and folate deficiency
Causes include pernicious anaemia, where autoimmune gastritis destroys the parietal cells that make intrinsic factor; loss of the terminal ileum; a strict vegan diet; and metformin. Folate deficiency can come fast, with poor diet, alcohol, pregnancy, or antifolate drugs such as methotrexate.
3:18Why our patient got worse
And that is why our patient got worse: folic acid can correct the anaemia while the nerve damage progresses. In megaloblastic anaemia, exclude B12 deficiency before giving folate alone.
3:31Your turn: practice question
Your turn. A 62-year-old man has had four months of numbness in his feet and an unsteady gait. Vibration and position sense are reduced in both legs. Haemoglobin is 9.4 g/dL and MCV is 112 femtolitres. Serum folate is normal. Which of the following is most likely abnormal in this patient, but normal in folate deficiency? A, elevated plasma homocysteine. B, elevated mean corpuscular volume. C, elevated serum methylmalonic acid. D, hypersegmented neutrophils on the blood smear. E, reduced serum folate. Pause here and decide. A is out: homocysteine rises in both deficiencies, because methionine synthase needs folate and B12. B is out: macrocytosis occurs in both. D is out: hypersegmented neutrophils also occur in both. E is out: his folate is stated to be normal. So the answer is C. Only B12 is needed by methylmalonyl-CoA mutase, so only B12 deficiency raises methylmalonic acid.
4:54High-yield recap and next lesson
Three things to keep. One: both deficiencies raise homocysteine and give the same megaloblastic anaemia. Two: methylmalonic acid rises only in B12 deficiency, which also damages the nervous system. Three: in megaloblastic anaemia, exclude B12 deficiency before giving folate alone. Next, intravascular versus extravascular haemolysis.
Sources
Numbers in the text, e.g. [1], point here- OpenStax Anatomy and Physiology 2e, 18.3 Erythrocytes. openstax.org/books/anatomy-and-physiology-2e/pages/18-3-erythrocytes
- MSD Manual Professional Version. Vitamin B12 Deficiency. msdmanuals.com/professional/nutritional-disorders/vitamin-deficiency,-dependency,-and-toxicity/vitamin-b12-deficiency
- MSD Manual Professional Version. Folate Deficiency. msdmanuals.com/professional/nutritional-disorders/vitamin-deficiency,-dependency,-and-toxicity/folate-deficiency
- Maynard 2023, folate and one-carbon metabolism. europepmc.org/article/PMC/PMC10147059
- Europe PMC review on vitamin B12 biochemistry and diagnosis. europepmc.org/article/PMC/PMC13653423
- Europe PMC review on megaloblastic anaemia. europepmc.org/article/PMC/PMC13508813
- Subacute combined degeneration overview. europepmc.org/article/PMC/PMC12468238
- StatPearls, Vitamin B12 Deficiency (PMID 28722952).
- MedlinePlus. Folate-deficiency anemia. medlineplus.gov/ency/article/000551.htm
The practice question is original and its patient is synthetic. Every fact is checked against the sources above in independent accuracy reviews. Always follow your local guidelines and your own laboratory’s ranges.
Image credits. Illustrations: Servier Medical Art (smart.servier.com), CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/), adapted (recoloured, outlined, labelled): erythrocyte, neutrophil granulocyte, mitochondria and others listed in the lesson page. Also used: Health Icons (CC0): vegetables, alcohol; Bioicons (CC0): pill by Marcel Tisch; public-domain NIH BioArt bone marrow. Original diagrams: the folate and methionine cycles, count curve, number line and store jars; tract overlays drawn by us on the Servier cord section. No AI-generated teaching images.
Corrections. None so far. Spotted an error? Email info@medservatory.com with the lesson code MS-S1-HEME-009.
For exam study only, not medical advice. Read the medical disclaimer.
In this series
Playlist on YouTube- EP 01Microcytic anaemia: four causes, four iron fingerprints5:37
- EP 02Hepcidin: why iron falls in inflammation5:34
- EP 03B12 vs folate deficiency: MMA, homocysteine and the nerves5:23
- EP 04Intravascular vs extravascular haemolysis: site and labs5:27
- EP 05G6PD deficiency vs hereditary spherocytosis, by mechanism5:36
- EP 06Sickle cell disease pathophysiology: HbS to vaso-occlusion5:26
- EP 07Primary haemostasis: platelet adhesion, activation and aggregation5:53
- EP 08Coagulation cascade: PT vs aPTT, pathway by pathway5:21
- EP 09Von Willebrand disease vs haemophilia: the labs that separate them5:32
- EP 10Anticoagulants: target, monitoring test and reversal agent5:34
- EP 11Heparin-induced thrombocytopenia: why a low count clots5:29
- EP 12DIC vs TTP vs HUS: mechanism and the PT/PTT5:33
- EP 13AML vs ALL vs CML vs CLL: leukemia classification made simple5:44
- EP 14Oncogenes vs tumour suppressor genes: one hit or two5:17