B Vitamins and the Brain: B12, Folate, and Beyond

Short answer: B12 or folate deficiency can cause irreversible nerve damage. But a meta-analysis of 11 trials in 22,000 people found B vitamin supplements did not slow cognitive decline. Vegans, adults over 50 and people on metformin or acid-reducing drugs are most at risk of low B12. Keep B6 supplements within the EU limit of 12 mg a day.

B vitamins matter for the brain mainly when you are short of them. Getting enough B12 and folate prevents serious nerve damage, but for people who are not deficient, B vitamin supplements have not protected memory or thinking in large trials. The evidence for a cognitive benefit is preliminary: one small trial found slower brain shrinkage in older adults with mild cognitive impairment and high homocysteine.

So the practical question is whether you are at risk of deficiency. If you are, food changes, fortified foods or a supplement chosen with your doctor are the answer. If you eat a varied diet including animal products, there is no evidence that routine B vitamin supplements improve cognition.

Who is at risk of B vitamin deficiency

Large US and UK surveys found about 6 percent of people aged 60 and over are B12 deficient (Allen 2009). Close to 20 percent have marginal status. Deficiency develops gradually, so nerve damage can progress for years before symptoms appear, and some of it may be irreversible.

• Older adults: In older people, poor absorption of B12 from food becomes the main cause of deficiency (Allen 2009). One reason is atrophic gastritis, in which the stomach makes less acid and intrinsic factor. B12 in food is bound to protein and needs stomach acid to be released. For this reason the National Academy of Medicine recommends that adults over 50 get most of their B12 from supplements or fortified foods, where it is in free form.
• Vegans: B12 is found only in animal foods, so vegans who don’t use a supplement or fortified foods are very likely to become deficient. The body stores B12 for several years, so it can take years to show. Every major nutrition and dietetics organization, including the Academy of Nutrition and Dietetics, recommends that vegans take a B12 supplement. Folate and B6 are plentiful in plant foods. More in our article on vegan diets and brain health.
• People taking metformin: The diabetes drug reduces B12 absorption. In a long-term follow-up of a randomized trial (Aroda 2016), each year of metformin use was linked to 13 percent higher odds of B12 deficiency. Borderline-low B12 was about twice as common in the metformin group at five years. The American Diabetes Association recommends periodic B12 checks for people on long-term metformin, but screening is inconsistent.
• People taking acid-reducing drugs: Proton pump inhibitors (PPIs) such as omeprazole, esomeprazole, lansoprazole and pantoprazole reduce the stomach acid needed to release B12 from food. In a large case-control study in California (Lam 2013), a two-year or longer supply of PPIs was linked to 65 percent higher odds of B12 deficiency.
• People with gut conditions or bariatric surgery: Celiac disease, Crohn’s disease, inflammatory bowel disease and bariatric surgery can reduce absorption of several B vitamins. Crohn’s often affects the end of the small intestine, where B12 is absorbed. Gastric bypass sharply reduces B12 and thiamine absorption, and lifelong monitoring and supplements are standard care afterwards.

If you take metformin or a PPI long term, ask your doctor about B12 testing.

How much you need and safe upper limits

B12. The RDA is the amount that prevents deficiency in healthy people with normal absorption. For older adults and people with absorption problems, doctors often use much higher oral doses, because only about 1 percent of a large dose is absorbed by passive diffusion. The dose depends on the cause, so treatment of deficiency should be guided by a doctor.

Folate. Cognitive trials have typically used 400 to 800 micrograms of folic acid. The US upper limit covers folic acid from supplements and fortified foods.

B6. Long-term high intake can cause peripheral neuropathy: nerve damage with numbness and tingling, which can mimic B12 deficiency. In 2023 the European Food Safety Authority set an adult upper limit of 12 mg a day (EFSA 2023).

EFSA based this on reports of neuropathy at doses around 50 mg a day. Many supplements, including some B-complex products, exceed 12 mg. Doses above the upper limit should only be taken under medical supervision.

Interactions. B vitamin supplements can interact with some medicines. For example, folic acid and B6 can affect some anticonvulsants, and B6 can reduce the effect of levodopa taken without carbidopa. Ask a doctor or pharmacist before starting a supplement, especially if you are pregnant, breastfeeding, take medication or have a health condition.

Methylated or standard forms?

Methylcobalamin is the form of B12 that the folate-recycling enzyme uses; cyanocobalamin is a synthetic form the body converts. Cyanocobalamin is more stable, better studied and cheaper, and most clinical trials used it. For most people it works well. Methylcobalamin may help people who convert poorly, but large trials showing it is better are lacking.

Methylfolate (5-MTHF) is folate in its active form. It skips the MTHFR enzyme, which in theory helps people with a common gene variant (see below). But folic acid is still converted in these people, just more slowly, and with adequate intake the difference is small for most people. One real advantage of methylfolate: it does not mask B12 deficiency the way high-dose folic acid can.

Methylated forms are reasonable and may offer small benefits for certain genotypes. Standard forms are effective, well tested and much cheaper. Neither choice is wrong.

Food sources

If you eat animal products, two to three servings of animal protein a day will generally meet B12 needs if your absorption is normal. Sardines and salmon supply both B12 and B6. Eat folate-rich greens and legumes daily. There is no reliable plant source of active B12; vegans need a supplement or fortified foods, and a dietitian or pharmacist can advise on the dose.

What the trials show for memory and thinking

Homocysteine: the link between B vitamins and brain aging

Folate, B12 and B6 together keep homocysteine in check. Homocysteine is an amino acid made during normal metabolism. Folate and B12 recycle it back to methionine; B6 sends it down another pathway. When any of the three runs short, homocysteine builds up.

High homocysteine (generally above about 15 micromol/L) is a consistently replicated risk factor for cognitive decline, brain shrinkage and dementia in observational studies. In 1,092 older adults from the original Framingham Study (Seshadri 2002), those above 14 micromol/L had nearly double the risk of Alzheimer’s disease over a median of eight years.

In the Hordaland Homocysteine Study in Norway, a population study of more than 18,000 people, older participants with raised homocysteine were more likely to have cognitive deficits (Refsum 2006). In lab studies, high homocysteine damages neurons and blood vessels through several pathways.

The VITACOG trial

The main positive trial is VITACOG at the University of Oxford (Smith 2010). It randomized 271 adults over 70 with mild cognitive impairment to high-dose B vitamins (0.8 mg folic acid, 0.5 mg B12 and 20 mg B6 daily) or a placebo. Of these, 168 had MRI scans at the start and after two years.

Brain volume fell by 0.76 percent a year on B vitamins against 1.08 percent on placebo, about 30 percent slower. In people who started with homocysteine above 13 micromol/L, the rate of shrinkage was 53 percent lower.

A follow-up analysis (Douaud 2013) found B vitamins reduced shrinkage in grey matter areas vulnerable to Alzheimer’s disease, including the medial temporal lobe. The effect was confined to people with homocysteine above the median (11 micromol/L).

On cognitive tests, B vitamins helped executive function overall, and helped memory and global cognition only in people with homocysteine above the median (de Jager 2012). A retrospective subgroup analysis (Jernerén 2015) found the treatment worked only in people with adequate omega-3 levels.

Some caveats. The investigators held patents on B vitamins for Alzheimer’s disease. Brain shrinkage is a scan measure, not a direct measure of memory or thinking. And the B6 dose (20 mg) is above the EU upper limit of 12 mg a day. The results suggest any benefit is limited to people with a metabolic shortfall to correct.

The larger trials were null

VITACOG was small. A meta-analysis (Clarke 2014) pooled 11 large trials with cognitive data on 22,000 people, treated for an average of several years. B vitamins lowered homocysteine by about a quarter but had no effect on any cognitive domain, on global cognitive function or on the rate of cognitive aging.

Most of these trials did not select people with high homocysteine or mild cognitive impairment, so they don’t rule out a benefit in that group. But they show that B vitamin supplements do not slow cognitive decline in the general older population. B vitamins do not treat dementia. Correcting a diagnosed B12 deficiency is important and can improve some symptoms.

What each B vitamin does in the brain

• B1 (thiamine): Needed by key enzymes that turn glucose into energy for neurons. Severe shortage causes Wernicke encephalopathy, a neurological emergency with confusion, unsteady walking and abnormal eye movements. Untreated, it can progress to Korsakoff syndrome, with severe memory loss. It is classically linked to heavy alcohol use but also occurs with malnutrition, after bariatric surgery, with severe pregnancy vomiting (hyperemesis gravidarum) and with very restricted eating.
• B3 (niacin): The body uses it to make NAD+, a coenzyme needed for energy production and DNA repair in neurons. Severe deficiency causes pellagra, whose symptoms include dermatitis, diarrhea and dementia. Pellagra is now rare in countries that fortify food. The body can also make some niacin from the amino acid tryptophan.
• B6 (pyridoxine): Its active form is needed to make serotonin, dopamine, norepinephrine, GABA and histamine. It also helps clear homocysteine. Deficiency can cause confusion, depression and weakened immunity.
• Folate (B9): Supplies the chemical groups used in many reactions that control genes and build cell membranes and neurotransmitters. Folic acid taken around conception greatly reduces the risk of neural tube defects such as spina bifida. In the MRC Vitamin Study (MRC 1991), a randomized trial in 1,817 women who had had an affected pregnancy, folic acid cut recurrence by 72 percent. In adults, folate deficiency causes anemia, raised homocysteine and a higher risk of depression. Low folate is consistently linked to poorer cognition in observational studies, especially in older adults.
• B12 (cobalamin): Works with folate to recycle homocysteine, and is needed to break down certain fats and amino acids. When B12 runs out, methylmalonic acid (MMA) builds up, which makes MMA a sensitive marker of deficiency. Deficiency can damage the spinal cord, causing numbness, tingling, unsteady walking and eventually paralysis if untreated. Memory loss, personality changes and dementia can also occur. Nerve damage can develop without anemia and, once established, may be only partly reversible.

MTHFR gene variants

MTHFR is the enzyme that converts folate into its active form. A common variant, C677T, makes the enzyme less active, more so in people with two copies (the TT genotype). TT is more common in some populations, including some Hispanic and southern Italian ones.

People with TT tend to have higher homocysteine, especially when folate intake is low. TT combined with low folate has been linked to neural tube defects, heart disease, depression and cognitive decline in observational studies. But this is not a reason for exotic treatment.

The key step is getting enough folate; with adequate intake, even people with TT can keep homocysteine normal. Methylfolate is sometimes recommended for them, but large trials have not shown it is better than folic acid in well-nourished people.

Frequently Asked Questions

Is it possible to get too much B12?

B12 has no set upper limit because no adverse effects have been seen even at very high doses. The kidneys clear the excess, and oral doses of 1,000 micrograms or more are commonly used in practice without safety concerns. Very high blood B12 not explained by supplements can occasionally point to liver disease, blood disorders or kidney failure, so a doctor should check it.

Does folic acid fortification mean nobody is low in folate?

No. US fortification of grain products began in 1998 and has cut neural tube defects and improved folate status. But people who eat little grain, have malabsorption or drink heavily may still be low. The fortification level was set to prevent neural tube defects, not to lower homocysteine or protect cognition.

How do I know if I have a B12 deficiency?

Symptoms can be vague: fatigue, weakness, numbness or tingling in the hands and feet, difficulty walking, memory problems, mood changes and a sore tongue. Diagnosis needs blood tests. Serum B12 levels in the “low normal” range (200-300 pg/mL) may already reflect a shortfall.

Raised MMA is highly specific for B12 deficiency; raised homocysteine can reflect low B12, folate or B6. If homocysteine is raised, your doctor will look for the cause, which may include low B12 or folate, kidney problems or medicines.

Sources

• Smith, A. D., Smith, S. M., de Jager, C. A., Whitbread, P., Johnston, C., Agacinski, G., Oulhaj, A., Bradley, K. M., Jacoby, R., & Refsum, H. (2010). Homocysteine-lowering by B vitamins slows the rate of accelerated brain atrophy in mild cognitive impairment: a randomized controlled trial. PLoS ONE, 5(9), e12244. doi:10.1371/journal.pone.0012244

• Douaud, G., Refsum, H., de Jager, C. A., Jacoby, R., Nichols, T. E., Smith, S. M., & Smith, A. D. (2013). Preventing Alzheimer’s disease-related gray matter atrophy by B-vitamin treatment. Proceedings of the National Academy of Sciences, 110(23), 9523–9528. doi:10.1073/pnas.1301816110

• de Jager, C. A., Oulhaj, A., Jacoby, R., Refsum, H., & Smith, A. D. (2012). Cognitive and clinical outcomes of homocysteine-lowering B-vitamin treatment in mild cognitive impairment: a randomized controlled trial. International Journal of Geriatric Psychiatry, 27(6), 592–600. doi:10.1002/gps.2758

• Jernerén, F., Elshorbagy, A. K., Oulhaj, A., Smith, S. M., Refsum, H., & Smith, A. D. (2015). Brain atrophy in cognitively impaired elderly: the importance of long-chain omega-3 fatty acids and B vitamin status in a randomized controlled trial. The American Journal of Clinical Nutrition, 102(1), 215–221. doi:10.3945/ajcn.114.103283

• Clarke, R., Bennett, D., Parish, S., Lewington, S., Skeaff, M., Eussen, S. J., … & Grodstein, F. (2014). Effects of homocysteine lowering with B vitamins on cognitive aging: meta-analysis of 11 trials with cognitive data on 22,000 individuals. The American Journal of Clinical Nutrition, 100(2), 657–666. doi:10.3945/ajcn.113.076349

• Seshadri, S., Beiser, A., Selhub, J., Jacques, P. F., Rosenberg, I. H., D’Agostino, R. B., Wilson, P. W., & Wolf, P. A. (2002). Plasma homocysteine as a risk factor for dementia and Alzheimer’s disease. The New England Journal of Medicine, 346(7), 476–483. doi:10.1056/NEJMoa011613

• Refsum, H., Nurk, E., Smith, A. D., Ueland, P. M., Gjesdal, C. G., Bjelland, I., … & Vollset, S. E. (2006). The Hordaland Homocysteine Study: a community-based study of homocysteine, its determinants, and associations with disease. The Journal of Nutrition, 136(6 Suppl), 1731S–1740S. doi:10.1093/jn/136.6.1731S

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• Aroda, V. R., Edelstein, S. L., Goldberg, R. B., Knowler, W. C., Marcovina, S. M., Orchard, T. J., … & Crandall, J. P. (2016). Long-term metformin use and vitamin B12 deficiency in the Diabetes Prevention Program Outcomes Study. The Journal of Clinical Endocrinology & Metabolism, 101(4), 1754–1761. doi:10.1210/jc.2015-3754

• Refsum, H., Smith, A. D., Ueland, P. M., Nexo, E., Clarke, R., McPartlin, J., … & Scott, J. M. (2004). Facts and recommendations about total homocysteine determinations: an expert opinion. Clinical Chemistry, 50(1), 3–32. doi:10.1373/clinchem.2003.021634

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• EFSA Panel on Nutrition, Novel Foods and Food Allergens (NDA), Turck, D., et al. (2023). Scientific opinion on the tolerable upper intake level for vitamin B6. EFSA Journal, 21(5), e08006. doi:10.2903/j.efsa.2023.8006