Vitamin B12 is necessary for making red blood cells and is important for the health of nerve cells and the formation of genetic material. Initial symptoms of vitamin B12 deficiency include dizziness and fatigue. Without treatment, deficiency can result in worsening anaemia, damage to the nervous system and other health issues.
Vitamin B12, also called cobalamin, is a water-soluble vitamin found in certain foods: meat, fish, milk products and eggs. Vegetables alone are an inadequate source of vitamin B12.
According to a New Zealand Adult Nutrition Survey, 8% of the population may be vitamin B12 deficient. The survey also revealed inadequate vitamin B12 levels are more common among females than males.
Vitamin B12 deficiency is most commonly caused by a lack of intrinsic factor - a substance produced in the stomach that is necessary for the absorption of vitamin B12 from food. Some people simply don't produce enough intrinsic factor while others may have a health condition that destroys it. Other causes of vitamin B12 deficiency include:
- Not eating vitamin B12-rich foods. This particularly affects vegans who don’t eat animal products (meat, fish, milk, eggs, butter, cheese and other dairy products)
- Poor absorption or use of B12 because of gastric problems such as coeliac disease, inflammation of the stomach (gastritis), or pancreas problems
- Poor absorption can also occur after stomach and intestinal surgery, and in inflammatory bowel diseases like Crohn's disease
- Drugs that may interfere with the absorption of vitamin B12
- Heavy alcohol consumption
- Rare congenital problems.
Many of the initial symptoms of vitamin B12 deficiency are associated with anaemia, where your body's cells don't get enough oxygen from your blood. Deficiency symptoms include
- Muscle weakness
- Shortness of breath
- Hearing difficulties.
If left untreated vitamin B12 deficiency can cause progressive damage to the nervous system - especially the nerves outside the brain and spinal cord. When the spinal cord is involved, the first symptoms include difficulty in feeling vibrations in the feet, loss of position sense, and loss of muscle co-ordination (ataxia). Other symptoms of untreated vitamin B12 deficiency may include:
- Weight loss
- Heart and blood vessel disease
- An enlarged spleen and liver
- Exaggerated reflexes
- Mild depression and confusion
- Hallucinations, personality and mood changes
- Damage to the optic nerve.
Once vitamin B12 deficiency has been diagnosed, a test that measures the absorption of vitamin B12 (the Schilling test) may be used to help detect possible underlying health conditions and treatment options.
In most cases of vitamin B12 deficiency, intramuscular injections of vitamin B12 are given. These injections bypass barriers to absorption of vitamin B12 from food. To begin with, regular injections of 1000 micrograms are usually given every week for 4-6 weeks. This frequency allows the body’s physiology to start producing red blood cells normally. It also allows the reserves that the liver normally holds to increase.
The injections do not solve the underlying cause of the deficiency and, depending on the underlying cause, it may be necessary to continue having B12 injections for life. It is usually recommended that a maintenance dose of 1000 micrograms is given every three months.
It is recommended that vegans take vitamin B12 supplements or vitamin B12 enriched nutritional yeast. Oral iron supplements can be prescribed if an iron deficiency is also present.
Side effects of vitamin B12 supplementation
Adverse effects resulting from vitamin B12 supplementation are rare. Hypersensitivity (anaphylaxis) is exceptionally rare and may include swelling, itching, and shock. Very high doses of vitamin B12 may sometimes cause acne. Other uncommon side effects include skin rash, hot flushes, nausea, dizziness and cardiac arrhythmias.
It is important to note that vitamin B12 supplements can be destroyed if taken within an hour of large amounts of vitamin C. Absorption can also be reduced by deficiencies in folic acid, iron or vitamin E. Use of nicotine or excessive alcohol can also deplete vitamin B12 levels.
Improved absorption of vitamin B12 occurs when it is taken with other B vitamins or calcium. Talk to your doctor about other possible interactions with medications you may use.
Mayo Clinic (2017). Vitamin B12 (Web Page). Rochester, NY: Mayo Foundation for Medical Education and Research. https://www.mayoclinic.org/drugs-supplements-vitamin-b12/art-20363663 [Accessed: 19/06/19]
National Institutes of Health Office of Dietary Supplements (2018). Vitamin B12 Fact Sheet (Web age). Bethesda, MD: National Institutes of Health. https://ods.od.nih.gov/factsheets/VitaminB12-HealthProfessional/ [Accessed: 19/06/19]
University of Otago and Ministry of Health. (2011). A Focus on Nutrition: Key findings of the 2008/09 New Zealand Adult Nutrition Survey. Chapter 4. Nutrient intakes and dietary sources: Micronutrients. Wellington: New Zealand Ministry of Health. http://www.health.govt.nz/system/files/documents/publications/a-focus-on-nutrition-ch4_0.pdf
O’Toole, M.T. (Ed.) (2017). Vitamin B12 test (cyanocobalamin). Mosby’s Dictionary of Medicine, Nursing & Health Professions (10th ed.). St Louis, MI: Elsevier.
Updated – July 2019