🚨 Never treat a suspected deficiency with folic acid alone without checking vitamin B12 first. Folate corrects the anaemia of B12 deficiency, so the blood count returns to normal and the problem appears solved, while the neurological damage of untreated B12 deficiency continues and can become permanent¹. This applies to high-dose folic acid supplements and to B-complex products containing large amounts of folate. It is placed first because it is the one thing on this page that can cause lasting harm.

Definition

What Are B12 and Folate Deficiency?

Both vitamins are needed to make DNA, so both are needed to make red blood cells. Running short of either produces large, immature red cells and the same anaemia, called megaloblastic anaemia. On the full blood count they can look identical.

They part company outside the bone marrow. B12 is also needed to maintain the myelin sheath around nerves, and prolonged deficiency causes a progressive neurological syndrome affecting sensation, balance, and eventually cognition. Folate does not do this.

And crucially, the neurological damage of B12 deficiency can occur before, or entirely without, any anaemia, which is why a normal blood count does not exclude it and why symptoms of numbness and unsteadiness deserve a B12 test rather than reassurance.

💡 Key Insight: The commonest cause of B12 deficiency is not a diet low in B12. It is failure to absorb it, from autoimmune loss of intrinsic factor, from age-related stomach changes, from bowel disease or surgery, or from long-term medicines. That matters because a deficiency of absorption is not fixed by eating more liver.

Illustration comparing normal red blood cells with the enlarged immature cells of megaloblastic anaemia, alongside a nerve fibre with a damaged myelin sheath

🧠 B12 protects nerves

Prolonged deficiency damages the spinal cord and peripheral nerves, producing numbness, pins and needles, unsteadiness and, later, cognitive change. Treated early it reverses. Left long enough it does not¹.

💊 Common medicines deplete it

Metformin is associated with lower B12 levels and with deficiency in long-term users². Long-term acid suppression reduces absorption too, since stomach acid is needed to release B12 from food.

🌱 Plants do not supply B12

B12 comes from animal foods and fortified products. Spirulina, algae and most seaweeds contain inactive analogues that do not work in humans and can make a blood test look falsely reassuring³.

Signs & Symptoms

How These Deficiencies Present

Slowly, vaguely, and often mistaken for ageing, stress or depression.

Feature	What it feels like	Worth knowing
Fatigue and breathlessness on exertion	The anaemia itself	Shared with iron deficiency, and the two can coexist
Numbness or pins and needles	Usually feet first, then hands, often symmetrical	A B12 sign, not a folate one. Report it promptly rather than waiting
Unsteadiness, worse in the dark	Balance depending on vision because position sense is impaired	Suggests spinal cord involvement. This is the presentation that must not be delayed
Sore, smooth, red tongue	Glossitis, sometimes with mouth ulcers	Occurs with both deficiencies
Memory difficulty, low mood, irritability	Often attributed to ageing or depression	B12 deficiency is one of the things worth excluding before concluding depression
Pale or slightly yellow skin	A lemon tinge, from anaemia plus mild jaundice	Reflects red cells being destroyed in the marrow
Symptoms with a NORMAL blood count	Neurological symptoms and no anaemia at all	Common, and the reason a normal full blood count does not exclude B12 deficiency
In pregnancy, folate matters differently	No symptoms; the risk is to the developing baby	Folate before conception and in early pregnancy reduces neural tube defects. Follow your maternity advice

⚠️ Who is most likely to be deficient. People over about 60, in whom stomach acid production falls. Vegans and strict vegetarians, and breastfed infants of vegan mothers. Anyone on long-term metformin² or long-term acid suppression; see GERD. People with coeliac disease, Crohn's disease or previous bowel or stomach surgery. Heavy drinkers, in whom folate deficiency is common. And people with autoimmune conditions, since pernicious anaemia travels with them; see thyroid disorders.

Assessment

How These Are Assessed

The important part is establishing WHY, because the cause determines whether tablets will work at all.

Measure	What it tells you	What it misses
Full blood count and blood film	Anaemia and large red cells	Can be entirely normal in B12 deficiency with neurological damage. Also normal-looking if iron deficiency coexists, since the two cell changes cancel out
Serum B12	The first-line test	An imperfect test with a grey zone. A borderline result in someone with symptoms deserves further testing rather than dismissal
Serum or red cell folate	Folate status	Serum folate moves with recent meals; red cell folate reflects longer-term status
Methylmalonic acid, and homocysteine	Functional markers that rise before stores are exhausted, useful in the grey zone	Not available everywhere, and MMA also rises in kidney impairment
Intrinsic factor antibodies	Pernicious anaemia, the autoimmune cause	Insensitive, so a negative result does not exclude it
Coeliac serology	A treatable malabsorptive cause	Must be done while still eating gluten; see coeliac disease
Medication review	Metformin and long-term acid suppression, both common and both reversible contributors	Requires someone to ask; a prescriber conversation, never a reason to stop a medicine yourself
Thyroid function	Autoimmune thyroid disease travels with pernicious anaemia	See thyroid disorders

💡 Ask for B12 and folate to be checked together, and ask what the cause is. If the cause is dietary, tablets work. If the cause is failure to absorb, as in pernicious anaemia or after stomach surgery, standard-dose tablets may not be enough and injections are usually used. Being told your level is low without being told why leaves you unable to know whether the treatment offered will actually fix it.

Treatment Comparison

Holistic vs. Conventional Treatment

🌿 HOLISTIC

💊 CONVENTIONAL

🌿

Holistic / Functional Approach

Food sources, reliable supplementation where diet cannot supply it, and fixing what is depleting you

Where Diet Works

Dietary deficiency, which responds fully to food or oral supplementation

Where It Does Not

Absorption failure. Pernicious anaemia and post-surgical deficiency are not corrected by eating differently

Timeline

Blood counts recover over weeks. Nerve symptoms improve over months, and only if treated before damage is established

Limitation, stated plainly

Nothing dietary treats pernicious anaemia, which needs lifelong medical replacement

Full Holistic Approach Includes

B12 from animal foods or reliable fortified products, since plants do not supply the active form³.
Folate from leafy greens, legumes, and fortified grains, which is easy to reach from food.
A reliable B12 supplement if you are vegan, which is not optional and is not covered by algae products.
Reviewing metformin and acid suppression with your prescriber, not stopping them yourself.
Reducing alcohol, which impairs folate absorption and use.
Treating the underlying gut condition where there is one.
Retesting, to confirm the correction actually happened rather than assuming.

🌿 Worth knowing: this is one of the few pages on this site where a nutrient genuinely is the treatment. The catch is that it only works when the cause is intake rather than absorption.

Nutritional Strategy

Diet for B12 and Folate

Folate is easy to get from food. B12 is not, if you do not eat animal products.

⚠️ If you follow a vegan diet, a reliable B12 source is not optional. B12 is produced by bacteria and reaches humans through animal foods or fortification. Spirulina, chlorella, most seaweeds and fermented foods contain corrinoid analogues that are inactive in humans, and worse, they can register on some blood tests and make a level look adequate when it is not³. Use a supplement or reliably fortified foods, and have your level checked. See the vegan diet guide.

Nutrient	Best food sources	Practical note
Vitamin B12	Meat, fish, shellfish, eggs, dairy, and fortified foods including some plant milks, nutritional yeast and breakfast cereals	No reliable plant source exists unfortified. Check labels rather than assuming a product is fortified
Folate	Leafy greens, legumes, beans, asparagus, broccoli, citrus, and fortified grains	Easy to reach from food. Prolonged boiling destroys a substantial share, so steam or use the cooking water
Both, together	Liver is exceptionally rich in both	Not in pregnancy, because of its very high preformed vitamin A content
Alcohol	—	Impairs folate absorption and metabolism, and is a common contributor to folate deficiency
Fortified foods	Cereals, plant milks, some yeast extracts	The practical route for vegans and for many older adults
Cooking method	—	Folate is heat and water sensitive; B12 is comparatively stable

💡 Age changes the advice. From around 50, stomach acid production commonly falls, and acid is needed to release B12 bound to food protein. B12 in supplements and fortified foods is not protein-bound, so it is absorbed more easily. That is why fortified foods or a supplement are often recommended for older adults even when their diet contains plenty of B12 on paper.

Supplement Protocol

Evidence-Based Supplements

The order in which you take these matters more than the dose.

🚨 Test B12 before starting folic acid, and test before starting B12 too. Starting B12 first makes a subsequent test uninterpretable, so if you suspect deficiency, get the blood test before the supplement rather than after. And never take high-dose folate to treat unexplained anaemia or fatigue without a B12 result in hand, for the reason at the top of this page¹. B-complex and multivitamin products frequently contain both, which usually protects you, but do not rely on that.

Supplement	What it is actually for	Typical range	Timing	Notes & cautions
Vitamin B12	Correcting a confirmed deficiency, and routine prevention for vegans and many older adults.	Set with your clinician. Higher oral doses are used where absorption is impaired, and injections where it fails	Daily, or as prescribed	Water-soluble and well tolerated. The form matters less than whether you can absorb it at all, which is a question about cause. Confirm the correction by retesting rather than assuming.
Folic acid	Correcting confirmed folate deficiency, and preconception and early pregnancy supplementation on maternity advice.	As directed by your clinician or maternity team	Daily	Only after B12 has been checked, outside of the pregnancy indication where it is standard advice. Also note folate interacts with methotrexate; on that medicine follow your rheumatology team, see rheumatoid arthritis, and some anticonvulsants lower folate.
Iron, only if also deficient	Correcting coexisting iron deficiency, which is common and can hide the large-cell picture on a blood count.	Test ferritin and full blood count first; dose set with your clinician	As advised, away from tea, coffee and calcium	Do not supplement iron without testing. See iron deficiency anaemia.

🚨 What to avoid, specifically. Algae, spirulina, chlorella and seaweed as a B12 source, which supply inactive analogues and can make a test look falsely adequate³. High-dose folic acid taken for tiredness without a B12 result. Assuming a B-complex covers it without checking the label. Stopping metformin or acid suppression yourself because you read they deplete B12; that is a prescriber conversation. And treating persistent numbness or unsteadiness as something to monitor rather than something to have assessed, because the window in which nerve damage reverses is limited.

💡 Where the real leverage is. Getting both tested together, finding out WHY rather than only that the level is low, treating B12 before folate, and retesting to confirm it worked. If you are vegan or over 60, a reliable B12 source taken routinely prevents the whole problem.

References & Evidence Notes

Each numbered entry below is either a source you can follow or a note setting out what the evidence does and does not support. Both are numbered together so the markers in the text line up.

Last reviewed 3 September 2026. Supplement entries are cross-checked against the NIH National Center for Complementary and Integrative Health and the Linus Pauling Institute Micronutrient Information Center.⁴ This page is nutrition education, not medical advice, and it does not replace your doctor. A low B12 or folate needs a cause, not just a supplement. Pernicious anaemia and other absorption failures require medical replacement for life and are not corrected by diet. Neurological symptoms need assessment promptly rather than a trial of tablets. What nutrition education can do is help you understand the evidence well enough to have a better conversation with the clinician who does treat you.

Green R, et al. Vitamin B12 deficiency. Nat Rev Dis Primers. 2017;3:17040. PubMed 28660890. The source for the neurological consequences of untreated deficiency, for the fact that neurological damage can occur without anaemia, and for the caution that folate can correct the haematological picture while neurological damage progresses. Also the basis for treating B12 before or alongside folate rather than after it. ↩ ↩ ↩ ↩
Metformin use and vitamin B12 deficiency in people with type 2 diabetes. touchREVIEWS in Endocrinology. 2024;20(2). PubMed 39526048. Long-term metformin use is associated with lower B12 levels and with deficiency, with risk rising with dose and duration. This is a reason to monitor, and a prescriber conversation, not a reason to stop a medicine that is treating diabetes. See diabetes. ↩ ↩
On algae and seaweed as B12 sources: spirulina, chlorella and most seaweeds contain corrinoid compounds that are structurally similar to B12 but biologically inactive in humans, and which can interfere with some assays so that a measured level appears adequate. See the NIH Office of Dietary Supplements vitamin B12 fact sheet, ods.od.nih.gov, and the Linus Pauling Institute entry on vitamin B12, lpi.oregonstate.edu. ↩ ↩ ↩ ↩
NIH National Center for Complementary and Integrative Health, nccih.nih.gov, and the Linus Pauling Institute Micronutrient Information Center, lpi.oregonstate.edu. ↩

B12 & Folate Deficiency