​What is vitamin B12?

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Cobalamin (Vitamin B12) is a vital nutrient your body relies on every day. It plays a key role in energy production, helps form red blood cells, supports DNA synthesis and keeps your nervous system healthy (S. Harikrishnan et al., 2024).

Most people get B12 from a balanced diet that includes animal-based foods like meat, fish, and dairy. Some of the richest sources are beef liver, clams or fortified products such as nutritional yeast. However, those following a vegan diet or with certain gastrointestinal conditions may need additional supplementation to prevent deficiency (NHS, 2020). 

 

What does it do within the body, and what happens without it?

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Your body can store B12 for several years (sometimes up to 5), but deficiency is not rare. Low levels of B12 or folate can lead to megaloblastic anaemia, whereby red blood cells become unusually large and less effective. B12 deficiency is diagnosed using a serum B12 concentration blood test, with levels below 148 pmol/L considered deficient (Obeid et al., 2024), although some individuals report symptoms above this threshold. 

 

Symptoms can range from headaches, fatigue, dizziness, pins and needles, cognitive decline or neuropathy (nerve damage or dysfunction) (NHS, 2020). Importantly, nerve damage caused by prolonged deficiency may become permanent. Research has also found patients dealing with Alzheimer's disease have lower vitamin B12 levels compared to healthy controls. Supplementation in those patients with mild cognitive impairment/dementia may experience a slower decline (Umekar et al., 2025). 

 

Several conditions can impair B12 absorption. One major cause is Pernicious anaemia, an autoimmune condition, that prevents the body from producing intrinsic factor (IF), a protein needed to absorb B12 (Behinger, Kulkarni and Weinstein 2025).

Other factors include:

Gastrointestinal diseases such as Crohn's disease, coeliac disease and long term use of certain medications, including proton pump inhibitors (PPI's) and metformin, a widely used glucose-lowering drug (Obeid et al., 2024). Research shows that people taking metformin may have a higher risk of developing nerve-related symptoms linked to B12 deficiency (Yang et al., 2023). 

 

A lesser know factor can be your genetics. Some people have a variation in the methylenetetrahydrofolate reductase gene (MTHFR), which affects how the body processes folate and related nutrients. This can indirectly impact B12 function, leading to imbalances and elevated homocysteine  levels which can be associated with an increased risk of cardiovascular disease, depression and other health concerns (Giammarco et al., 2024). 

 

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This process also depends on folate (vitamin B9). Without enough B12, this partnership breaks down leading to problems such as impaired DNA production and development of large, immature red blood cells as seen in megaloblastic anaemia (Mazur et al., 2025).

 

​Testing and treatment

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B12 deficiency is typically identified through a routine blood test measuring serum B12 and folate levels.

Treatment depends on the cause and severity of the deficiency: 

Absorption issues - (for example pernicious anaemia), B12 injections of cyanocobalamin and methyl cobalamin are needed (Obeid et al., 2024).

Diet-related deficiency - Oral B12 supplements or fortified foods maybe enough.

Alternative options - Some patients use sublingual (under the tongue) drops as these bypass parts of the digestive system (Mazur et al.,2025).

Adults need 1.5 micrograms per day.

 

Who is most at risk?

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While B12 deficiency can affect anyone, certain groups are particularly vulnerable:

• Vegans and vegetarians — as B12 is found almost exclusively in animal-based foods, those following plant-based diets are at significantly higher risk and should consider regular supplementation or consumption of fortified foods

• Older adults — absorption efficiency tends to decline with age, partly due to reduced stomach acid production, which is needed to release B12 from food proteins

• People with gastrointestinal conditions — those living with Crohn's disease, coeliac disease, or those who have had gastric surgery may have impaired absorption regardless of dietary intake

• People taking certain medications — long-term use of metformin or proton pump inhibitors (PPIs) is associated with reduced B12 levels and should be monitored

• Those with pernicious anaemia — this autoimmune condition prevents the production of intrinsic factor, making it impossible to absorb B12 through diet alone

• Those with an MTHFR gene mutation — this relatively common genetic variant can impair the body's ability to process folate and B12 effectively

If you fall into any of these categories, it is worth discussing B12 testing with your GP, even in the absence of obvious symptoms.

 

When should you see a GP?

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Because B12 deficiency can develop slowly and its symptoms overlap with many other conditions, it is often missed or misattributed. You should seek medical advice if you experience any of the following:

• Persistent fatigue or weakness that does not improve with rest

• Tingling, numbness, or pins and needles in the hands or feet

• Memory problems, difficulty concentrating, or mood changes

• Dizziness or shortness of breath

• Pale or jaundiced skin

• A sore or inflamed tongue

Do not wait for symptoms to become severe. Because nerve damage from B12 deficiency can be irreversible, early diagnosis and treatment is important. A routine blood test is all that is needed to check your levels.

 

This article is intended for informational purposes only and does not constitute medical advice. Always consult a qualified healthcare professional before making changes to your diet, supplementation, or treatment plan.

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How can I help?

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As a Registered Nutritionist, I can help you identify potential dietary gaps, understand your symptoms, and create a personalised plan to support healthy vitamin B12 levels through food, supplementation and lifestyle guidance. Small dietary changes can make a significant difference to your energy, brain function and overall well being.

I'd love to support you ​

🥑EXPLORE MY SERVICES

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Behringer, C.R., Kulkarni, A. and Weinstein, A. (2025). Vitamin B12: A Comprehensive Review of Natural vs Synthetic Forms of Consumption and Supplementation. Cureus. [online] doi:https://doi.org/10.7759/cureus.96258.

Giammarco, S., Patrizia Chiusolo, Maggi, R., Rossi, M., Minnella, G., Elisabetta Metafuni, D’Alò, F. and Sica, S. (2024). MTHFR polymorphisms and vitamin B12 deficiency: correlation between mthfr polymorphisms and clinical and laboratory findings. Annals of Hematology. doi:https://doi.org/10.1007/s00277-024-05937-z.

Kannan R;Ng MJ (2015). Cutaneous lesions and vitamin B12 deficiency: an often-forgotten link. Canadian family physician Medecin de famille canadien, [online] 54(4). Available at: https://pubmed.ncbi.nlm.nih.gov/18413300/.

Kumar, S., Vijayan, J., Jacob, J., Alexander, M., Gnanamuthu, C. and Aaron, S. (2005). Clinical and laboratory features and response to treatment in patients presenting with vitamin B12 deficiency-related neurological syndromes. Neurology India, 53(1), p.55. doi:https://doi.org/10.4103/0028-3886.15057.

Mazur, M., Artnora Ndokaj, Salerno, C., Vallone, J., Ardan, R., Bietolini, S., Carrouel, F., Wilk, A., Sarig, R., Ottolenghi, L. and Bourgeois, D. (2025). Efficacy of sublingual and oral vitamin B12 versus intramuscular administration: insights from a systematic review and meta-analysis. Frontiers in Pharmacology, 16, pp.1602976–1602976. doi:https://doi.org/10.3389/fphar.2025.1602976.

NHS (2020). B Vitamins and Folic Acid - Vitamins and Minerals. [online] NHS. Available at: https://www.nhs.uk/conditions/vitamins-and-minerals/vitamin-b/.

Obeid, R., Andrès, E., Češka, R., Hooshmand, B., Guéant-Rodriguez, R.-M., Prada, G.I., Sławek, J., Traykov, L., Ta Van, B., Várkonyi, T., Reiners, K. and The Vitamin B12 Consensus Panelists Group (2024). Diagnosis, Treatment and Long-Term Management of Vitamin B12 Deficiency in Adults: A Delphi Expert Consensus. Journal of Clinical Medicine, [online] 13(8), p.2176. doi:https://doi.org/10.3390/jcm13082176.

S Harikrishnan, Kaushik, D., Kumar, M., Kaur, J., Oz, E., Charalampos Proestos, Tahra Elobeid, Omur Fatih Karakullukcu and Oz, F. (2024). Vitamin B12: prevention of human beings from lethal diseases and its food application. Journal of the Science of Food and Agriculture. doi:https://doi.org/10.1002/jsfa.13661.

Umekar, M., Premchandani, T., Tatode, A., Qutub, M., Raut, N., Taksande, J. and Hussain, U.Md. (2025). Vitamin B12 deficiency and cognitive impairment: A comprehensive review of neurological impact. Brain Disorders, [online] 18, p.100220. doi:https://doi.org/10.1016/j.dscb.2025.100220.

Yang, R., Yu, H., Wu, J., Chen, H., Wang, M., Wang, S., Qin, X., Wu, T., Wu, Y. and Hu, Y. (2023). Metformin treatment and risk of diabetic peripheral neuropathy in patients with type 2 diabetes mellitus in Beijing, China. Frontiers in Endocrinology, 14. doi:https://doi.org/10.3389/fendo.2023.1082720.