Vitamin B12 and IQ: Can Low B12 Affect Intelligence?

Severe vitamin B12 deficiency in infancy can damage the developing brain and has been linked to lasting intellectual problems, but in people who are not severely deficient, B12 supplements do not raise IQ. The best evidence is a randomised trial in Nepal that gave B12 to 600 infants at risk of deficiency for a year and found no benefit on a full-scale IQ test at age 3 to 4: the B12 group scored half a point lower than the placebo group, a difference indistinguishable from zero.

This page covers who becomes deficient, what deficiency does to children's development, what the supplementation trials in children and older adults found, and where homocysteine fits in. It describes research and is not medical advice.

Vitamin B12 (cobalamin) is needed to make red blood cells and to keep the nervous system healthy, including the fatty myelin sheath that insulates nerve fibres. It comes almost entirely from animal foods such as meat, fish, eggs and dairy, plus fortified foods and supplements.

The UK's National Health Service lists the main routes to deficiency. The most common cause in the UK is pernicious anaemia, an autoimmune condition that stops the gut absorbing B12. A vegan diet without supplements or fortified foods can also lead to it, as can certain medicines. Deficiency of B12 or folate becomes more common with age, affecting around 1 in 20 people aged 65 to 74 and 1 in 10 aged 75 or over.

The diabetes drug metformin is another known cause. In the long-running Diabetes Prevention Program, people assigned to metformin were more likely than those on placebo to have low or borderline-low B12 after five years (19.1% versus 9.5%), and the risk rose with each year of use.

Infants are a special case. A baby's B12 stores come from the mother during pregnancy and then from breast milk, so infants of mothers with untreated pernicious anaemia or a strict vegan diet without supplements can become deficient even when the mother feels well.

A 2008 review by Dror and Allen pooled published case reports of infants with severe B12 deficiency born to mothers with untreated pernicious anaemia or a strict vegetarian diet. The pattern is consistent: irritability, poor feeding, failure to thrive, apathy and loss of skills the baby already had. The symptoms respond remarkably quickly to treatment.

Quick recovery does not always mean full recovery. A 1992 case series by Graham and colleagues described six infants with dietary B12 deficiency. Four could be followed up years later, and two of those four had a poor intellectual outcome. That is a tiny sample of the most severe cases, and it cannot say how common lasting harm is, but it is a strong reason to catch infant deficiency early.

Milder deficiency in childhood has also been studied. In the Netherlands, Louwman and colleagues tested 48 adolescents who had eaten a macrobiotic (vegan-type) diet until about age 6 and 24 who had always eaten an omnivorous diet. Of the macrobiotic group, 31 still showed biochemical signs of B12 deficiency years after the diet had changed. Deficiency correlated with lower scores on a test of fluid intelligence (r = -0.28), and the correlation was stronger within the macrobiotic group (r = -0.38). The study has real limits. It was observational, and the omnivorous controls outscored the macrobiotic children on most tests whether or not those children were deficient, which suggests that family background differed between the groups as well as diet.

Observational links can reflect poverty, illness or general undernutrition, so randomised trials are the decisive test. Their results are mostly null.

• North India: Kvestad and colleagues gave 422 children aged 6 to 30 months B12, folic acid, both, or placebo for six months. Children who got both vitamins scored 0.28 standard deviations higher on problem solving (about 4 IQ-point equivalents) and 0.45 higher on gross motor skills than the placebo group. B12 alone helped gross motor scores only. The outcome was a parent-supported developmental questionnaire, which is a much rougher measure than an IQ test.

• Nepal, infancy: Strand and colleagues randomised 600 infants aged 6 to 11 months who were already slightly short for their age, and therefore at risk of poor nutrition, to B12 or placebo for a year. B12 improved blood markers of B12 status but had no effect on the Bayley infant development scales, growth or haemoglobin.

• Nepal, preschool follow-up: when the same children were 42 to 47 months old, Ulak and colleagues gave 533 of them the Wechsler Preschool and Primary Scale of Intelligence (WPPSI-IV), a standard IQ test for young children. Full-scale IQ averaged 84.4 in the B12 group and 85.0 in the placebo group, a difference of half a point with a 95% confidence interval from about 2 points lower to 1 point higher. No subgroup benefited.

• Nepal, pregnancy: Chandyo and colleagues gave 800 pregnant women B12 or placebo from early pregnancy until six months after birth. Most (71%) had low or marginal B12 at the start. Supplementation improved the mothers' B12 status, but the infants' Bayley cognitive scores, which use the same mean of 100 and standard deviation of 15 as IQ, differed by only 0.5 points.

Put together, these trials suggest that topping up B12 in children who are marginally low, in places where marginal status is common, does not raise intelligence. They do not test what happens to a child with severe clinical deficiency, because withholding treatment from such a child would be unethical. The wider picture of food and children's development is in our overview of whether diet or nutrition affects IQ. Iron is a useful contrast: there, randomised trials in deficient children do show gains, as covered in our page on iron deficiency and IQ.

In older adults the question is usually about memory and decline. Low B12 raises blood levels of "homocysteine", an amino acid that builds up when B12 or folate is short. High homocysteine is a risk factor for dementia, which made B vitamins an attractive target.

The trials have mostly disappointed:

• Older adults with mild deficiency: Eussen and colleagues randomised 195 people aged 70 or over with mild B12 deficiency to B12, B12 plus folic acid, or placebo for 24 weeks. The supplements corrected the deficiency but did not improve attention, memory, speed or executive function. Memory actually improved slightly more in the placebo group than in the B12-only group.

• The largest pooled analysis: Clarke and colleagues combined 11 trials with about 22,000 participants. B vitamins lowered homocysteine by about a quarter but had no effect on any cognitive domain or on global cognition. On the main domain measure the difference was 0.00 standard deviations, with a confidence interval that rules out anything larger than about 1 IQ-point equivalent.

• A 2021 meta-analysis: Markun and colleagues pooled 16 trials with 6,276 participants who did not have overt deficiency or advanced neurological disease, and found no effect of B12 alone or B-complex vitamins on any area of cognition.

The VITACOG trial is often cited as the counterexample. Smith and colleagues gave 271 people over 70 with mild cognitive impairment high-dose folic acid, B6 and B12 or a placebo for two years. Among the 168 who completed brain scans, the brain shrank 0.76% a year on vitamins against 1.08% on placebo, and the slowing was 53% in people who started with high homocysteine. That is a result about brain atrophy in a high-risk group, and brain size itself is only weakly related to IQ. It is not evidence that B12 raises IQ.

Confounds run through all of the observational B12 research. Diet tracks income, education and culture, illness and poor appetite can cause both low B12 and poor cognition, and memory problems can change what people eat, so low B12 may sometimes be a consequence of decline as well as a cause.

Not in people who are not severely deficient. In a randomised trial of 600 Nepalese infants, a year of B12 made no difference to full-scale IQ at age 3 to 4, and trials in older adults show no cognitive benefit.

Severe deficiency in infancy can cause developmental regression, and small case series report lasting intellectual problems in some children. Milder deficiency has been linked to lower fluid reasoning scores in observational studies, which cannot rule out other differences between families.

Infants of mothers who eat a strict vegan diet without B12 supplements or fortified foods are at risk, because the baby relies on the mother's B12. Parents should discuss this with a midwife, GP or paediatrician, ideally during pregnancy.

Large trials and meta-analyses find no improvement in memory or other cognitive functions, even in people with mild deficiency. Diagnosed deficiency still needs treatment for other reasons, including nerve damage and anaemia.

A GP or family doctor, who can order a blood test and look for the cause. The NHS advises prompt diagnosis because some problems caused by deficiency can be irreversible.

Vitamin B12 matters for the brain, and severe deficiency in a baby is a medical emergency with possible lasting effects on development. That does not make B12 an IQ supplement. Randomised trials in at-risk infants, pregnant women and older adults all found that extra B12 improved blood levels without improving intelligence or cognition. If you suspect deficiency, see a doctor; if you are curious about your reasoning ability, take an online IQ test built by psychometricians.

1. National Health Service. (n.d.). Vitamin B12 or folate deficiency anaemia. NHS. [nhs.uk](. nhs.uk

2. Aroda, V. R., Edelstein, S. L., Goldberg, R. B., Knowler, W. C., Marcovina, S. M., Orchard, T. J., Bray, G. A., Schade, D. S., Temprosa, M. G., White, N. H., Crandall, J. P., & the Diabetes Prevention Program Research Group. (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.org/10.1210/jc.2015-3754](. doi.org

3. Dror, D. K., & Allen, L. H. (2008). Effect of vitamin B12 deficiency on neurodevelopment in infants: Current knowledge and possible mechanisms. Nutrition Reviews, 66(5), 250-255. [doi.org/10.1111/j.1753-4887.2008.00031.x](. doi.org

4. Graham, S. M., Arvela, O. M., & Wise, G. A. (1992). Long-term neurologic consequences of nutritional vitamin B12 deficiency in infants. The Journal of Pediatrics, 121(5), 710-714. [doi.org/10.1016/S0022-3476(05)81897-9](. doi.org

5. Louwman, M. W., van Dusseldorp, M., van de Vijver, F. J., Thomas, C. M., Schneede, J., Ueland, P. M., Refsum, H., & van Staveren, W. A. (2000). Signs of impaired cognitive function in adolescents with marginal cobalamin status. The American Journal of Clinical Nutrition, 72(3), 762-769. [doi.org/10.1093/ajcn/72.3.762](. doi.org

6. Kvestad, I., Taneja, S., Kumar, T., Hysing, M., Refsum, H., Yajnik, C. S., Bhandari, N., Strand, T. A., & Folate and Vitamin B12 Study Group. (2015). Vitamin B12 and folic acid improve gross motor and problem-solving skills in young North Indian children: A randomized placebo-controlled trial. PLOS ONE, 10(6), e0129915. [doi.org/10.1371/journal.pone.0129915](. doi.org

7. Strand, T. A., Ulak, M., Hysing, M., Ranjitkar, S., Kvestad, I., Shrestha, M., Ueland, P. M., McCann, A., Shrestha, P. S., Shrestha, L. S., & Chandyo, R. K. (2020). Effects of vitamin B12 supplementation on neurodevelopment and growth in Nepalese infants: A randomized controlled trial. PLOS Medicine, 17(12), e1003430. [doi.org/10.1371/journal.pmed.1003430](. doi.org

8. Ulak, M., Kvestad, I., Chandyo, R. K., Ranjitkar, S., Hysing, M., Schwinger, C., Shrestha, M., Basnet, S., Shrestha, L. P., & Strand, T. A. (2023). The effect of infant vitamin B12 supplementation on neurodevelopment: A follow-up of a randomised placebo-controlled trial in Nepal. British Journal of Nutrition, 129(1), 41-48. [doi.org/10.1017/S0007114522000071](. doi.org

9. Chandyo, R. K., Kvestad, I., Ulak, M., Ranjitkar, S., Hysing, M., Shrestha, M., Schwinger, C., McCann, A., Ueland, P. M., Basnet, S., Shrestha, L., & Strand, T. A. (2023). The effect of vitamin B12 supplementation during pregnancy on infant growth and development in Nepal: A community-based, double-blind, randomised, placebo-controlled trial. The Lancet, 401(10387), 1508-1517. [doi.org/10.1016/S0140-6736(23)00346-X](. doi.org

10. Eussen, S. J., de Groot, L. C., Joosten, L. W., Bloo, R. J., Clarke, R., Ueland, P. M., Schneede, J., Blom, H. J., Hoefnagels, W. H., & van Staveren, W. A. (2006). Effect of oral vitamin B-12 with or without folic acid on cognitive function in older people with mild vitamin B-12 deficiency: A randomized, placebo-controlled trial. The American Journal of Clinical Nutrition, 84(2), 361-370. [doi.org/10.1093/ajcn/84.2.361](. doi.org

11. Clarke, R., Bennett, D., Parish, S., Lewington, S., Skeaff, M., Eussen, S. J., Lewerin, C., Stott, D. J., Armitage, J., Hankey, G. J., Lonn, E., Spence, J. D., Galan, P., de Groot, L. C., Halsey, J., Dangour, A. D., Collins, R., & 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.org/10.3945/ajcn.113.076349](. doi.org

12. Markun, S., Gravestock, I., Jäger, L., Rosemann, T., Pichierri, G., & Burgstaller, J. M. (2021). Effects of vitamin B12 supplementation on cognitive function, depressive symptoms, and fatigue: A systematic review, meta-analysis, and meta-regression. Nutrients, 13(3), 923. [doi.org/10.3390/nu13030923](. doi.org

13. 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.org/10.1371/journal.pone.0012244](. doi.org

Hero image: Tisrio Tistryo (Clams), by Joegoaukfishcurry2, CC BY-SA 2.0. Via Wikimedia Commons.