Last updated by Dr Sarah Brewer on
Vitamin B12 consists of a group of chemicals known as cobalamins, such as methylcobalamin, hydroxocobalamin and cyanocobalamin. These vitamins all contain cobalt within their structure, which is the only known requirement for cobalt in the human body. Vitamin B12 is important for energy production in cells, and to prevent a type of anaemia called pernicious anaemia because it tends to creep up on you, unnoticed, as stores of vitamin B12 are slowly depleted.
Vitamin B12 supplements are used to treat tiredness, promote sleep, and may reduce the risk of heart disease and dementia.
Vitamin B12 benefits
Vitamin B12 is needed by enzymes involved in amino acid metabolism. Cells with a rapid turnover have the highest requirement, such as those lining the gut (which are shed every three days), hair follicle cells and the bone marrow which continually produces new red blood cells.
When vitamin B12 is in short supply, newly formed cells are larger than usual. This is most noticeable when it affects red blood cells, resulting in pernicious anaemia.
Good supplies of vitamin B12 are also important during pregnancy and, together with folic acid, vitamin B12 helps to protect against some developmental abnormalities such as spina bifida.
Vitamin B12 is involved in processing homocysteine – a potentially harmful amino acid which can damage artery linings and hasten hardening of the arteries (atherosclerosis) when allowed to build up. Vitamin B12 is also needed for healthy nerve function, immunity and healing.
Within the EU, the European Food Safety Authority has authorised health claims that vitamin B12 contributes to:
- Normal energy-yielding metabolism
- Normal functioning of the nervous system
- Normal homocysteine metabolism
- Normal psychological function
- Normal red blood cell formation
- Normal function of the immune system
- The reduction of tiredness and fatigue
- The process of cell division.
Vegetarians, especially those following a vegan diet, are at risk of vitamin B12 deficiency since it is mainly found in animal based foods such as:
- Offal, especially liver
- Meat, especially lamb
- Oily fish, especially sardines and salmon
- Dairy products
Vegetarian sources of vitamin B12 include blue-green algae, extracts from bacterial cultures, and a synthetic vitamin B12 which is found in fortified breakfast cereals.
Around 20% of the B12 in food leaches out into juices during cooking.
Unlike most water-soluble vitamins, vitamin B12 can be stored in the liver with enough stocks to last for several years.
Vitamin B12 is absorbed in the lower part of the small intestine when a carrier protein, called intrinsic factor, is present. Intrinsic factor is produced in the stomach by the parietal cells which also secrete hydrochloric acid. These cells tend to reduce in number with age (atrophic gastritis) with reduced levels of stomach acid and reduced production of intrinsic factor affecting as many as one in five people over the age of 60.
People with autoimmune conditions such as type 1 diabetes or thyroid disease are especially likely to develop B12 deficiency associated with autoimmune attack against intrinsic factor function.
Vitamin B12 deficiency can also result from poor absorption due to bowel disease such as Crohn’s disease, ulcerative colitis, or coeliac disease, and following surgery to remove parts of the stomach or small intestine.
Interestingly, absorption of vitamin B12 (when attached to intrinsic factor) is dependent on the presence of calcium, and some evidence suggests that calcium supplements can improve vitamin B12 absorption – at least in people with malabsorption due to the diabetes drug, metformin.
Prolonged vitamin B12 deficiency can lead to spinal cord damage (sub-acute, combined degeneration of the cord) although this is rare.
Symptoms that may be due to vitamin B12 deficiency
- smooth, sore tongue
- menstrual disorders
- reduced immunity.
Symptoms that may be due to major vitamin B12 deficiency
- difficulty walking, especially in the dark
- poor memory
- lack of concentration
Vitamin B12 and pregnancy
Vitamin B12 protects against a type of developmental abnormality known as neural tube defects, which include spina bifida and hydrocephalus. While the highest level of protection occurs with taking folic acid supplements, the addition of vitamin B12 in prenatal and pregnancy formulas increases the level of protection from taking folic acid supplements alone. It is therefore included in many pregnancy and preconception supplements.
Vitamin B12 and homocysteine
Raised blood levels of the amino acid, homocysteine, increase the risk of developing coronary heart disease and stroke. Normally, its level is tightly controlled by three different enzymes that convert homocysteine to cysteine – a safe end product used by cells for growth.
When vitamin B12 and/or folic acid are lacking, this conversion is inefficient and homocysteine build up in the circulation. Supplements of folic acid plus B12 can lower a raised homocysteine level especially in older people. Although studies show that vitamin B-12 can reduce homocysteine levels by 7%, in addition to the effects of folic acid supplements, and may help to reduce the risk of stroke.
Vitamin B12 and dementia
Raised homocysteine levels have been associated with cognitive impairment and dementia in later life. Low blood levels of vitamin B12 are also associated with cognitive decline and dementia. People with Alzheimer’s dementia also tend to have lower levels of number micronutrients, including vitamin B12.
However, trials in which people were given supplements of vitamin B12, B6 and folic acid, alone or in combination, to lower homocysteine levels did not appear to improve cognitive function in individuals with (or without) existing cognitive impairment. A study from Oxford University did find, however, that combining omega-3 fish oils with B vitamins in people with Alzheimer’s might slow their disease progression. In those taking high-dose B vitamins (folic acid 0.8 mg, vitamin B6 20 mg, vitamin B12, 0.5 mg) for two years, the rate of brain atrophy slowed by 40% compared with those taking placebo but only in those who also had high blood levels of omega-3 fatty acids. In those who had a raised homocysteine level the rate of brain shrinkage was more dramatic with a 70% reduction in its progression. No significant effects were seen in those with low omega-3 levels, suggesting that fish oils plus B vitamins are both needed.
Vitamin B12 and eye disease
Raised levels of homocysteine are linked with an increased risk of eye disease (proliferative retinopathy) in people with diabetes. Macular degeneration – another leading cause of blindness – is also associated with raised homocysteine levels and taking supplements containing folic acid and vitamin B12 may protect against this, too.
Vitamin B12 and sleep
Some studies suggest that high dose vitamin B12 (particularly methyl cobalamin) may influence biological rhythms and melatonin secretion, to have a beneficial effect on sleep disorders.
Vitamin B12 dose
Requirements increase during pregnancy and lactation.
The average Western diet supplies 6.2 mcg per day.
Multivitamins and B complex supplements tend to supply vitamin B12 in amounts ranging from 1mcg to 100mcg or more. Single supplements containing a high dose of 1000mcg (1mg) vitamin B12 are also available.
Although vitamin B12 treatment for pernicious anaemia traditionally involves intramuscular injections, high doses can be used for sub-lingual absorption (under the tongue) in high doses to overcome the lack of intestinal intrinsic factor.
Vitamin B12 safety
Itchy rash and diarrhoea have been reported from very high doses.
The European Food Safety Authority felt unable to determine a tolerable upper intake for long-term use from supplements due to a lack of evidence relating to toxicity.
However, for guidance purposes only, the UK Expert Group on Vitamins and Minerals suggested an upper tolerable intake of 2000mcg (2 mg).
Vitamin B12 deficiency can be masked by taking folate/folic acid so the two supplements are often combined.
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